ISSN:0003-2654    

DOI:10.1039/AN97398FX029

出版商:RSC    

年代:1973

数据来源: RSC

ISSN:0003-2654    

DOI:10.1039/AN97398BX031

出版商:RSC    

年代:1973

数据来源: RSC

摘要:

August, 19731 SUMMARIES OF PAPERS I N THIS ISSUESummaries of Papers in this IssueMass and Charge Transfer Kinetics and Coulometric CurrentEfficienciesPart VII. Conditional Potentials, and Single- scan Voltammetryof Pure Vanadium(V) - Vanadium(1V) Systems in VariousMedia at Platinum Electrodes Pre- treated by Five MethodsThe limited previous work on vanadium is reviewed. Five methods ofelectrode pre-treatment have been selected and are described. The variationof the conditional potential of the vanadium(V) - vanadiurn(1V) systemwith hydrogen-ion concentration is reported. The experimental work iscomplicated by isopolymerisation reactions, of which a t least one is kineti-cally slow. The voltammetric reduction of vanadium (V) in saturated potas-sium sulphate - acetate buffer a t pH 4-0 is examined: the benefit of shiitingsolvent reaction potentials to more negative potentials is nullified by thedirect reduction of un-ionised acetic acid.M sulphuricacid and in one of 2.0 M sulpliuric acid.Possible adsorption effects arecanvassed. The complex behaviour a t intermediate hydrogen-ion concen-trations is discussed, with illustrations drawn from a sulphuric acid mediumof pH 2.0. It isconcluded that the electrochemical behaviour of the vanadium system isstrongly dependent 011 hydrogen-ion concentration and on the electrodepre-treatment. The electrode can be chemically oxidised in vanadium (V)solutions. The mechanism is a one-step one-electron process. No evidencecould be found for reduction below the +4 oxidation state a t platinum.Vanadium( IV) cannot be oxidised without severe loss of current efficiency,nor reduced to vanadium(II1) at platinum electrodes.E.BISHOP and P. H. HITCHCOCKChemistry Department, University of Exeter, Stocker Road, Exeter, EX4 4QD.Analyst, 1973, 98, 553-562.A similar examination is made in a medium of 5 xThe anodic oxidation of vanadium(1V) is briefly examined.Mass and Charge Transfer Kinetics and Coulometric CurrentEfficienciesin the Presence of Chromium, Manganese and Iron, and the KineticParameters of the Vanadium System, at Platinum ElectrodesPre-treated by Five MethodsContinuing the earlier examination of the vanadium system alone, undervarious conditions and with various electrode pre-treatments, the effect ofneighbouring steel-forming d-block elements has been investigated. Chro-mium(V1) at pH 4-0 suppresses the vanadium(V) reduction wave, and thedegree of suppression is quantitatively proportional to the chromium (VI)concentration. Activated electrodes are deactivated by dipping them ina chromium(V1) solution, and remain so even when well washed thereafter,so that chromium(V1) as well as chromium(II1) is adsorbed strongly onplatinum.In 2.0 M sulphuric acid, chromium(V1) and vanadium(V) arereduced at the same rate. Manganese(VI1) in acetate buffer gives a fast,well separated wave, but the separation is not as good in 2.0 M sulphuric acid;slowing the vanadium(V) reduction by using an oxidised electrode effects noimprovement : the manganese wave is similarly affected.Addition ofchromium(V1) to the manganese - vanadium mixture at pH 4 suppressesthe manganese wave only slightly, even when the vanadium wave is com-pletely suppressed. In 2.0 M sulphuric acid, the manganese wave is un-distorted and chromium and vanadium are simultaneously reduced. Iron(II1)in 2-0 M sulphuric acid does not interfere, but the separation of the vanadiumand iron waves is not good. Iron(I1) can, however, act as a potentiostaticintermediate. The kinetic parameters of the vanadium system are repro-ducible in acetate buffer, but only when the electrode is fouled in 2.0 Msulphuric acid. Pattern theory and diffusion-corrected Lewartowicz methodsgive results that agree. The charge-transfer kinetic parameters are shownto be potential dependent in acidic media.The results are compared withthose in earlier reports. The generation current efficiency for vanadium (IV)in acetate buffer was computed.E. BISHOP and P. H. HITCHCOCKChemistry Department, University of Exeter, Stocker Road, Exeter, EX4 4QD.Analyst, 1973, 98, 563-571.Part VIII. Single-scan Voltammetry of Vanadium(V) - Vanadium(1V)...11iV THE ANALYST [August, 1973THE ANALYSTE D I T 0 R I AL AD VI S 0 RY BOARDChairman: H. J. Cluley (Wembley)*L. S. Bark (Salford)R. Belcher (Birmingham)t. J. Bellamy, C.B.E. (Waltham Abbey)L. S. Birks (U.S.A.)E. Bishop (Exeter)E. A. M. F. Dahmen (The Netherlandsj*J. B. Dawson (Leeds)A. C. Docherty (BillinghamjD. Dyrssen (Sweden)*W.T. Elwell (Birmingham)*D. C. Garratt (London)*R. Goulden (Sittingbourne)J. Hoste (Belgium)D. N. Hume (U.S.A.)H. M. N. H. Irving (Leeds)A. G. Jones (Welwyn Garden City)M. T. KeIley (U.S.A.)*J. A. Hunter (Edinburgh)W. Kemula (Poland)*G. F. Kirkbright (London)G. W. C. Milner (Harwell)G. H. Morrison (U.S.A.)*J. M. Ottaway (Glasgow)*G. E. Penketh (Billingham)S. A. Price (Tadworth)D. I. Rees (London)E. B. Sandell (U.S.A.)*R. Sawyer (London)A. A. Smales, O.B.E. (Harwell)H. E. Stagg (Manchester)E. Stahl (Germany)A. Walsh (Australia)T. S. West (London)P. Zuman (U.S.A.)*A. Townshend (Birmingham)* Members of the Board serving on the Executive Committee.NOTICE TO SUBSCRIBERS(other than Members of the Society)Subscriptions for The Analyst, Analytical Abstracts and Proceedings should beThe Chemical Society, Publications Sales Office,B I ac k h orse Road, Letc h wort h, He rts.Rates for 1973(a) The Analyst, Analytical Abstracts, and Proceedings, with indexes .. . . f37.00(b) The Analyst, Analytical Abstracts printed on one side of the paper (withoutindex), and Proceedings . . . . . . . . . . . . . . €38.00(c) The Analyst, Analytical Abstracts printed on one side of the paper (withindex), and Proceedings . . . . . . . . . . . . . . €45.00sent to:The Analyst and Analytical Abstracts without Proceedings-(d) The Analyst and Analytical Abstracts, with indexes . . . . . . . . €34.00(e) The Analyst, and Anolyticol Abstracts printed on one side of the paper (without(f) The Analyst, and Analytical Abstracts printed on one side of the paper (withindex) .. . . . . . . . . . . . . . . . . f35.00index) . . . . . . . . . . . . . . . . . . €42.00(Subscriptions are NOT accepted for The Analyst and/or for Proceedings aloneAugust, 19731 THE ANALYSTWhat didYOU say?You don’tsubscribeto AnalyticalChemistry?Then you’re missing out on one ofthe greatest bargains in chemicalI iteratu re!ANALYTICAL CHEMISTRY is theforemost publication in the vital fieldof chemical analysis. It’s the one,dependable, accurate source of in- 4and qualitative analysis. ‘For only $5.00 a year (if you are anACS member) and $7.00 (if you arenot) you’ll get fourteen valuable thickissues.In addition to the monthly issues,you’ll receive the 500-page “LAB-ORATORY GUIDE to INSTRUMENTS,EQUIPMENT and CHEMICALS” andthe special April ANNUAL REVIEWS.The ANNUAL REVIEWS alternatesbetween APPLICATIONS one yearBecause we’re so sure of ANALYTI-CAL CHEMISTRY’S value, we’ll giveyou this guarantee.Should it fail tomeet your expectations, we’ll behappy to refund the unused portionof your subscription money, at anytime.Fair enough?Just fill out and return the form.. and FUNDAMENTALS the next,Another ACS ServiceI Washington, D.C. 20036Please send me ANALYTICAL CHEMISTRY at the following subscription rate:OtherCanada PUAS Nations u. s.0 $5.00 0 $ 9.00 0 $ 9.00 0 $10.000 $7.00 0 $11.00 0 $19.00 $20.00ACS membersNonmembersNote: SubscriPtiOnS at ACS Member Rates are for personal use only.Analytical ChemistryAmerican Chemical Societb11 55 Sixteenth Street.N.W. I73-ACCI Name I AddressCity State/Country Zip I Your Nature of Company’sCompany Business0 I am an ACS member 0 I am not an ACS member Bill company 00 Bill me for $- Payment enclosed in the amount of $-(payable to American Chemical SocietySUMMARIES OF PAPERS I N THIS ISSUEManganese and Vanadium - Iron Mixtures and the Influenceof Chromium on the ProcessEarlier voltammetric work permitted the mass and charge transferkinetic parameters of the vanadium system to be calculated for diverse mediaand platinum electrode prc-treatments, and command potentials to beselected for potentiostatic determination of vanadium alone and in certaincombinations with other steel-forming elements. A simple coulometric celland an adaptation of a commercial potentiostat are described.Currentintegration by strip-chart recorder is too inaccurate and so RC integrationis discussed. Philbrick SP456 amplifiers refused capacitive feedback, butvery satisfactory results were given by Solartron AA 1023 amplifiers. Thedesign of a very high quality integrating capacitor from S.T.C. polystyreneelements is described ; leakage and drift tests were very satisfactory. Pre-treatments of electrolytes and electrodes are discussed. Vanadium (V) isdetermined a t -0.128 V in acetate buffer and at +O-247 V in 2.0 M sulpliuricacid, in the latter with a relative standard deviation of 0.27 per cent.anda 95 per cent. confidence level result of (1-008 to 1.011) x 10-1 M comparedwith 1.012 x 10-1 M for a standard solution. Chromium(VT) suppresses allreduction at pH 4.0, and is reduced simultaneously with vanadium in sul-phuric acid. Manganese(VI1) is reduced to manganese(II1) in the first stepat + 0-7 V a t pH 3.5 and manganese(II1) and vanadium (V) are simultaneouslyreduced in the second step at -0.12 V. The separation of iron(II1) is possiblea t +O-9 V but impracticable; simultaneous reduction a t +0.25 V in 2.0 Msulphuric acid followed by re-oxidation of the iron(I1) a t + 1.0 V is reconi-mended.E. BISHOP and P. H. HITCHCOCKChemistry Department, University of Exeter, Stocker Road, ISxeter, EX4 4QU.Analyst, 1973, 98, 572-579.[August, 1973Potentiostatic Coulometric Determination of Vanadium, Vanadium -A Multi-channel Dispenser - Titrator - pH-statA rnulti-channel dispenser - ti trator - pH-stat with switch-selection ofreagents and reagent volume control and with a directly digital read-out isdescribed.A valve driven by a stepper motor is used to select reagents andas a coarse volume control. A drop generator dispensing - deflection mechan-ism is used as a fine volume control. The drop generator is also used to rejectautomatically to waste any reagent contaminated by previously dispensedreagent and, in the titration mode, to add various amounts of reagent tothe reaction mixture. The instrument gives outstanding precision (relativestandard deviation less than 0.2 per cent.over the range 0.5 to 7-0 ml) andlinearity (correlation coefficient Y = 0.999 over the ranges 0.01 to 1.0 and0.25 to 7-0 nil).DOUGLAS G. MITCHELL and KENNETH M. ALDOUSDivision of Laboratories and Research, New York State Department of Health,Albany, New York 12201, U.S.A.Analyst, 1973, 98, 580-584.A Modified Field Test for the Determination of CarbonDisulphide Vapour in AirAn improved and more sensitive method is described for the determinationof carbon disulphide vapour in air a t concentrations up to 40 p.p.m. V / V .Carbon disulphide vapour is absorbed from a 500-ml sample of air into anetlianolic solution containing copper(I1) acetate, diethylamine and triethanol-amine. The yellow colour produced is compared visually with standardcolours or measured spectrophotometrically.For field use, the apparatus isportable and simple to operate, and requires a working time of about 5 minutesper determination.E. C. HUNT, W. A. McNALLY and A. F. SMITHDepartment of Trade and Industry, Laboratory of thc Govcrnnicnt Chemist,Cornwall House, Stamford Strcet, London, SE1 9NQ.Analyst, 1973, 98, 685-592August, 19731 SUMMARIES OF PAPERS I N T H I S ISSUEDetermination of the Antioxidant: 1,3,5-Trimethyl-2,4,6- tri(3’,5’-di- t- butyl- 4’- hydroxybenzy1)benzene in Feeds1,3,5-TrimethyI-2,4,6- tri (3’, 5’-cli- t-buty!-4’-liydrox?ibcnzyl) benzene (Ion-ox 380) is an antioxidant that is uscd for the prcscrvation of plastic foodwrappings. Its use as a fat stabiliser could Le extended to animal feeds.By using the method described, 200 p.p.m.of Ionox 330 in a feed can bedetermined with good reproducibility. The mcthod consists in extractingthe Ionox 330 with chlorofornl, purifying i t by using thin-layer chromato-graphy, and measuring a t 522 nm the colour developed in the presence ofiron(l1 I) chloride and 2,2’-bipyridyl. Possible interference from 2,G-cli-t-butyl-p-cresol (butylated hydroxytoluene; BHT) can be avoided by using this chroni-atograpliic technjque.G. F. BORIESStation Centrale de Nutrition, Centre National de Recherches Zootechniques,78- Jouy-en- Josas, France.Analyst, 1973, 98, 593-595.The Determination of Lead in Foods by Atomic- absorptionSpectrophotometryRapid procedures for the determination of lead in foods by an organicextraction technique and atomic-absorption spectrophotometry are described.The food sample can be dry ashed or digested by using sulphuric acid andhydrogen peroxide. In the latter instance, digestion need not be complete.Lead is extracted from acidic solutions (either the dissolved ashes or theresidual solution after acid digestion) into xylene as its dicthylamnioniumdiethyldithiocarbamate chelate, and then determined by use of atomic-absorption spectrophotometry .Large amounts of iron and tin do notinterfere in the determination. In a 10-g sample, 0.02 p.p.m. o€ lead canbe detected. The standard deviation in the range froni 0.2 to 1.0 p.p.m. oflead is about 0.02 p.p.m. Certain products do not require preliminary diges-tion; in these instances lead is extracted directly from the acidified sample.Liquids, beverages and many canned foods can be nionitored very rapidlyin this way.The chelate - solvent combination used in this method is moreconvenient than the ammonium tetramethylenedithiocarbamate - isobutylmethyl ketone system. The method is applicable also to metals other thanlead ; its use for cadmium has been demonstrated successfully.R. K. ROSCHNIKNest16 Products Technical Assistance Co. Ltd., Control Laboratory, Case Postale 88,Ch-1814, La Tour-de-Peilz, Switzerland.Analyst, 1973, 98, 596-604.A Critical Study of Safranine 0 as a Spectrophotometric Reagent :a Rapid Method for the Determination of Trace Amounts ofAntimony in SteelThe behaviour of Safranine 0 (Basic red 2, C.I.50240) in aqueous solutionsbas been invcstigated and its suitability as a spectrophotonietric reagent iscvaluated and discussed. ,4 rapid method for the determination of antimonyas a hexachloroantimonate(V) ion-association complex with Safranine 0 isdescribed. This complex is extracted into benzene and is determined spectro-photometrically. The method is as sensitive and reproducible as the Rhod-aniine B method but it has the advantage that the determination can becarried out directly on solutions of steel without prior separation. Goodagreement was found with standardised antimony values for five BritishChemical Standards’ steels.C. BURGESS, A. G. FOGG and D. THORBURN BURNSDepartment of Chemistry, University of Technology, Loughborough, Lcicestershire,LEll 3TU.Analyst, 1973, 98, 605-609...V l l l THE ANALYST [August, 1973SPECIALIST ABSTRACTJOURNALSpublished bySCIENCE AND TECHNOLOGY AGENCYAtomic Absorption and FlameEmission Spectroscopy AbstractsVol.5, 1973, bimonthly S30X-Ray Fluorescence SpectrometryAbstractsVol. 4, 1973, quarterly €28Thin-Layer Chromatography AbstractsVol. 3, 1973, bimonthly €28Gas Chromatography-MassSpectrometry AbstractsVol. 4, 1973, quarterly €37Nuclear Magnetic ResonanceSpectrometry AbstractsVol. 3, 1973, bimonthly S30Laser-Raman Spectroscopy AbstractsVol. 2, 1973, quarterly it30X-Ray Diffraction AbstractsVol. 1-2, 1973, quarterly €30Neutron Activation Analysis AbstractsVol. 2-3, 1973, quarterly €30Electron Microscopy AbstractsVol. 1,1973, quarterly €30Liquid Chromatography AbstractsVol. 1, 1973, quarterly €30Electron Spin Resonance SpectroscopyAbstractsVol. 1, 1973, quarterly €30Sample copies on request from:SCIENCE AND TECHNOLOGY AGENCY,3 HARRINGTON ROAD,SOUTH KENSINGTON,LONDON, SW7 3ES01-584 8081INDI-TABSTablets of pre-mixed chemicals. A simpleand economical way of adding indicators toyour titration flask.One hundred tablets each of twelve differentindicators are contained in each of two kitsfor titration of Aluminium, Bismuth,Cadmium, Calcium, Cobalt, Copper, Magne-sium, Nickel, Zinc, etc. etc.Details on request from:-RIDSDALE & CO. LTD.suppliers of “Analoid” compressed chemicalreagentsNewham Hall, Newby,Middlesbrough, Teesside, TS8 9EATel. Middlesbrough 0642 37216Why not use“COLLOID TITRATION”for simple and rapid determination ofCOLLOID PARTICLES in the field ofAgar and Alginic Acid IndustryLeather IndustryFood IndustryPaper and Pulp IndustryS u rfactant I n d u st ryWater Treatmentfor detailed brochure and reagentswrite soon to-DOJINDO CO., LTD.Research LaboratoriesP.O. Box 41, KumamotohigashiKumamoto (862)JAPANTel ex-7627-76 D 0 J I N

ISSN:0003-2654    

DOI:10.1039/AN97398FP085

出版商:RSC    

年代:1973

数据来源: RSC

摘要:

August, 19731 THE ANALYST ixCLASSIFIED ADVERTISEMENTSThe rate for classified aditcrtiscments is 35p a line (or spacerguivalrnt of a line) with a n extra charge of l o p f o r theuse of a 1jo.x Nzimbrr. Semi-displayed classifiedridvertiszrnents are L4 for single-column inch.Copy for classified advertisements required not later thavvthe 18tlz of thr month precedzng date of Publication whicha s o n the 16th o,! each month. Adoertasements should beaddressed to J . Arthur Cook, 9 Lloyd Square, London,W C I X 9B24. Tel.: 01-837 6315WANTEDAutomatic ariiino acid analyscr, secondhand, i n good working orderwanted urgently. Replies to Box No. 2 3 2 , c/o J. A r t h u r Cook, 9Lloyd Square, London WC1X 9BA.Please mentionTHE ANALYSTwhen replying to advertisementsAPPOINTMENTS VACANTAutoanalysis - Food IndustryHeinz, leaders i n the convenience sector of the food industry,require a young graduate, preferably with some experience incontinuous flow autoanalysis, to carry out development work onfoodstuffs analysis by this technique.The person appointed must have sufficient experience and initiativeto perform this task with minimum supervision.The Heinz Research Centre is situated in a pleasant parklandsetting in Hayes, Middlesex, and assistance in relocation can beoffered if necessary.Salary, working conditions and fringe benefitsare appropriate to a major food company.Please write in the first instance giving a brief personal and careersummary and quoting present salary to:-Mu. K. S.Chivers,Manager-H.O. Personnel Services,H. J. Heinz Company Ltd.,Hayes Park, Hayes,Middlesex UB4 8ALx THE ANALYST [August, 1973APPOINTMENTS VACANTThe Technical Services Division of Johnson &Johnson Ltd. require a forward thinking youngman aged 25 years or under who has the abilityto cope with general analytical work over awide range of products manufactured in theU.K.Applicants must have obtained a degree orequivalent in Chemistry and should preferablyhave had some experience of analytical workbut this is not essential.The modern laboratories are located in apleasant area on the South Coast and generousassistance will be given towards the cost ofrehousing where applicable. Salary will benegotiable.tions and career to:The Employee Relations Manager, Johnson &Johnson Ltd., Southampton Road, Cosham,Portsmouth, PO6 4RL, Hants.Applicants should write stating age, qualifica-Senior AnalystRoche Products Limited have an opening for alSenior Analyst to be responsible forthe analytical laboratory work in their Dalry, Ayrshire factory.This laboratoryprovides quality control information on raw materials and final products in syntheticvitamin and drug manufactures at this site; it also offers some analytical service tothe plant control and investigation laboratories.Applicants must therefore have at least five years ’ postgraduate training and experiencein this field (i.e. Ph.D. level) preferably in pharmaceutical chemistry, and have hadexperience in control of staff.The person appointed will be required to take chargeof a team of some 16 assistants.Write in confidence, quoting reference TJ, for an application form and furtherinformation, to the Director of PersonnelRoche Products Limited15 Manchester Square London W1M 6AAugust, 19731 THE ANALYST xiAPPOINTMENTS VACANTHampshire CoastCyanamid, part of an internationalpharmaceutical manufacturingorganisation, invite applications for theposition of Analytical Chemist withinthe Analytical Services Group of ourRegional Laboratory locatedhere in Gosport.The Regional Laboratory provides aservice to subsidiary companies, inEurope and Africa, with the AnalyticalServices Group being particularlyresponsible for the development andapplication of analytical proceduresrelating to our products and those ofour overseas subsidiary companies.The responsibilities of the successfulcandidate will include the applicationand evaluation of existing procedures,allied to the implementation of newprocedures where found necessary.The ability to maintain a high level ofoverall project efficiency will be allimportant to the total group function.Ideally we are looking for someonewho has recently obtained a degree inChemistry or who currently holds apost-graduate qualification inAnalytical Chemistry.An attractive salary will be offered, withnormal company benefits applying.Please write, enclosing curriculum vitae,or telephone for an application form to :Mr.T. Misselbrook,Personnel Officer,CYANAMID OF GREATBRITAIN LTD.,Fareham Road,Gosport, Hants.Telephone Fareham 6131Have your back numbers of The Analyst bound in the standardbinding case.Send t h e parts and the appropriate index(es) together with aremittance for f2.40 to:HEFFERS PRINTERS LTD, Cambridge, Englanxii THE ANALYST [August, 1973APPOINTMENTS VACANTDEPUTY PUBLIC ANALYSTANDDEPUTY 0 F F I C I A L AG R I C U LTU RAL AN A LY STE365444143 p.a.(P.O.l 6-10) plus S144 London WeightingApplications for the above post are invited from Associates ofthe Royal Institute of Chemistry holding the M.Chem.A. Diploma.It is essential that the person appointed has extensive seniorexperience in the work of a public analyst and agriculturalanalyst’s laboratory.Assistance with removal expenses in approved cases.Application forms from Personnel and Management ServicesDivision, 27 Peckham Road, S.E.5.(Tel. 01-701 2870, 24 hourrecordacall service). Reference No. A/12/4485. Closing date14th September, 1973.Now AvailableANALYTICAL SCIENCES MONOGRAPHNo. 1High-Precision Titrimetryby C. Woodward and H. N. RedmanImperial Chemical Industries Limited (Agricultural Division)BRIEF CONTENTS:IntroductionVisual Titrations, with sections on Apparatus, Standard Substances and their preparationInstrumental Methods, with sections on Photometric Titrations, Electrometric Titrationsand assay, and Standard Solutions.and Miscellaneous Techniques.References t o the literature of high-precision titrimetry.Pp.viii+63 Price f2.50Obtainable from :Society for Analytical Chemistry, Book Department,9/ 10 Savile Row, London, W I X I AFMembers may buy personal copies a t t h e special price of L2.0August, 19731 SUMMARIES OF PAPERS I N THIS ISSUEAn Emanation Method for Determining Radium UsingLiquid Scintillation CountingA very simple emanation method for determining radium is described.Radon is adsorbed on silica gel at the temperature of liquid nitrogen andthe silica gel is transferred a t 0 "C to a toluene-based liquid scintillator forcounting in an automatic liquid scintillation spectrometer. The lowest limitof detection is 0.1 pCi of radon.K. G. DARRALL, P. J. RICHARDSON and J. F. C. TYLERDepartment of Trade and Industry, Laboratory of the Government Chemist,Cornwall House, Stamford Street, London, SE1 9NQ.An~lyst, 1973, 98, 610-615.xiiiApplication of Gas - Liquid Chromatography to the Analysisof Essential OilsPart 11.Determination of 1,s-Cineole in Oils of Cardamom,Rosemary, Sage and Spike LavenderReport prepared by the Essential Oils Sub-committee.ANALYTICAL METHODS COMMITTEE9/10 Savile Row, London, W1X 1AF.Analyst, 1973, 98, 6113-623.SELECTED ANNUAL REVIEWSof theANALYTICAL SCIENCESVolume 2 - 1972CONTENTSThe Techniques and Theory of Thermal Analysis Applied to Studies on InorganicMaterials with Particular Reference to Dehydration and Single Oxide Systems -D. DollimoreDevelopments in Ion Exchange - F. VernonThermometric and Enthalpimetric Titrimetry - L.S. Bark, P. Bate and J. K. GrimeObtainable from-Pp. vi + 149 f5.00; U.S. $13.00 ISBN 0 85990 202 1The Society for Analytical Chemistry, Book Department,9/10 Savile Row, London Wl X I AFMembers of The Chemical Society may buy personal copies at the special price of f3.00; U.S. $8.0THE ANALYST [August, 1973 xivReprints of Review PapersREPRINTS of the following Review Papers published in The Analyst since January, 1963, areavailable from The Society for Analytical Chemistry, Book Department, 9/10 Savile Iioiv, London,W1X 1AF (not through Trade Agents). Orders MUST be accompanied by a remittance for thecorrect amount made out to “Society for Analytical Chemistry.”“Classification of Methods for Determining Particle Size, ’’ by the Particle Size Analysis“Methods of Separation of Long-chain Unsaturated Fatty Acids,” by A.T. James (August,“Beer’s Law and its Use in Analysis,” by G. F. Lothian (September, 1963).“A Review of the Methods Available for the Detection and Determination of Small Amounts“Circular Dichroism,” by R. D. Gillard (November, 1963).“Information Retrieval in the Analytical Laboratory,” by D. R. Curry (hTovember, 1963).“Thermogravimetric -4nalysis,” by A. TV. Coats and J . P. Redfern (December, 1963). Price“Some Analytical Problems Involved in Determining the Structure of Proteins and Peptides, ”“The Faraday Effect, Magnetic Rotatory Dispersion and Magnetic Circular Dichroism, ” by“Electrophoresis in Stabilizing Media,” by D. Gross (July, 1965).“Recent Developments in the Measurement of n’ucleic Acids in Biological Materials,” by“Radioisotope X-ray Spectrometry,” by J .R. Rhodes (November, 1966).“The Determination of Iron(I1) Oxide in Silicate and Refractory Materials,” by H. N. S.“Activation Analysis,” by R. F. Coleman and T. B. Pierce (January, 1967).“Techniques in Gas Chromatography. Choice of Solid Supports,” by F. J. Palframan“Heterocyclic Azo Dyestuffs in Analytical Chemistry,” by R. G. Anderson and G. A-ickless“Determination of Residues of Organophosphorus Pesticides in Food,” by D. C. Abbott and“Radioactive Tracer Methods in Inorganic Trace Analysis : Recent Advances,” by J . TV.“Gamma-activation Analysis,” by C. A. Baker (October, 1967).“Precipitation from Homogeneous Solution,” by P. F.S. Cartwright, E. J , Ncwman andD. W. Wilson (November, 1967).“Industrial Gas Analysis,” by (the late) H. N. W7ilson and G. M. S. Duff (December, 1967).Price 35p.“The Application of Atomic-absorption Spectrophotometry to the -4nalysis of Iron andSteel,’’ by P. H. Scholes (April, 1968).“Inorganic Ion Exchange in Organic and Aqueous - Organic Solvents,” by G. J . Moody andJ . D. R. Thomas (September, 1968).“Radiometric Methods for the Determination of Fluorine,” by J. I<. Foreman (June, 1969).Price 25p.“Techniques in Gas Chromatography. Developments in the van Deemter KateTheory of Column Performance,” by E. A. Walker and J. F. Palframan (August, 1969).Price 25p.Choice of Detectors,” by T. A. Gough andE. A. Walker (January, 1970).Sub-committee of the Analytical Methods Committee (March, 1963).1963).Price 25p.Price 25p.Price 25p.of Cyanide,” by L. S. Bark and H. G. Higson (October, 1963). Price 25p.Price 15p.Price 15p.25p.by Derek G. Smyth and D. F. Elliott (February, 1964).J . G. Dawber (December, 1964).Price 25p.Price 25p.Price 25p.H. N. Munro and A. Fleck (February, 1966). Price 25p.Price 25p.Schafer (December, 1966). Price 25p.Price 25p.Part I.and E. A. Walker (February, 1967).(April, 1967). Price 25p.H. Egan (August, 1967).McMillan (September, 1967). Price 25p.Price 25p.Price 25p.Price 25p.Price 25p.Price 25p.Price 35p.Part 11.“Techniques in Gas Chromatography.“Laser Raman Spectroscopy,” by P. J . Hendra and C. J . Vear (April, 1970).“Ion-selective Membrane Electrodes,” by Ern0 Pungor and IilAra T6th (July, 1970). Price“X-ray Fluorescence Analysis,” by I<. G. Carr-Brion and I<. W. Payne (December, 1970).“Mass Spectrometry for the Analysis of Organic Compounds,” by A. E. VC’illiams and H. E.“The Application of Non-flame Atom Cells in Atomic-absorption and Atomic-fluorescence“Liquid Scintillation Counting as an Analytical Tool,” by J. A. B. Gibson and A. E. LallyPart 111.Price 25p.Price 35p.35p.Price 25p.Stagg (January, 1971). Price 35p.Spectroscopy,” by G. F. Kirkbright (September, 1971).(October, 1971). Price 25p.Price 25p

ISSN:0003-2654    

DOI:10.1039/AN97398BP091

出版商:RSC    

年代:1973

数据来源: RSC

摘要:

AUGUST, 1973 THE ANALYST Vol. 98, No. 1169 Mass and Charge Transfer Kinetics and Coulometric Current Efficiencies Part VII.* Conditional Potentials, and Single-scan Voltammetry of Pure Vanadium(V) - Vanadium( IV) Systems in Various Media at Platinum Electrodes Pre-treated by Five Methodsf. BY E. BISHOP AND P. H. HITCHCOCKf (Chemistry Department, University of Exetev, Stockev Road, Exeter, E X 4 4QD) The limited previous work on vanadium is reviewed. Five methods of electrode pre-treatment have been selected and are described. The variation of the conditional potential of the vanadium(V) - vanadium (IV) system with hydrogen-ion concentration is reported. The experimental work is complicated by isopolymerisation reactions, of which a t least one is kineti- cally slow. The voltammetric reduction of vanadium (V) in saturated potas- sium sulphate - acetate buffer a t pH 4.0 is examined: the benefit of shifting solvent reaction potentials to more negative potentials is nullified by the direct reduction of un-ionised acetic acid.M sulphuric acid and in one of 2.0 M sulphuric acid. Possible adsorption effects are canvassed. The complex behaviour a t intermediate hydrogen-ion concen- trations is discussed, with illustrations drawn from a sulphuric acid medium of pH 2.0. The anodic oxidation of vanadium(1V) is briefly examined. It is concluded that the electrochemical behaviour of the vanadium system is strongly dependent on hydrogen-ion concentration and on the electrode pre-treatment. The electrode can be chemically oxidised in vanadium (V) solutions.The mechanism is a one-step one-electron process. No evidence could be found for reduction below the +4 oxidation state a t platinum. Vanadium(1V) cannot be oxidised without severe loss of current efficiency, nor reduced to vanadium(II1) a t platinum electrodes. IJNGANE~ could obtain little information from a polarographic investigation of the vana- dium(V) - vanadium(1V) system. The complex structure of the voltammetric waves at carbon electrodes in sulphuric and orthophosphoric acids defied interpretation,2 although the repro- ducibility was fairly good. Davis334 examined the reaction chronopotentiometrically and voltammetrically, and concluded that the reduction of vanadium(V) was accelerated by a lightly oxidised platinum surface (the oxygen bridge theory, since rejected5y6).Amperostatic determinations of vanadium(V) have been reported for copper(I),' iron(II),s t i t a n i u ~ n ( I I I ) , ~ ~ ~ ~ and tin( 11)5 as intermediates, Potentiostatic determination at a mercury cathode has been reported by Israel and Meites,ll but no report of reduction at platinum has appeared, although Davis12 remarked that the reaction was slow. A voltammetric study of the system has been made in a variety of media with a view to the determination of the mass and charge transfer kinetic parameters, and the selection of conditions for the potentiostatic determination of vanadium both alone and in mixtures with other steel alloying elements. EXPERIMENTAL A similar examination is made in a medium of 5 x General apparatus and procedures have been described earlier,l3 and electrode treatments and behaviour have been reviewed.14 The reagents were of the highest purity available (Aristar, P.V.S.or AnalaR grade) and were further purified if necessary. Sodium perchlorate was made from sodium carbonate and perchloric acid. Presented a t the Second SAC Conference, Nottingham, 1968. * For particulars of Part VI of this series, see reference list, p. 562; for Part VIII, see p. 563. Z Present address: Ever Ready Co. (G.B.) Ltd., Central Research Laboratories, St. Ann's Road, @ SAC and the authors. London, N15 3TJ. 553554 BISHOP AND HITCHCOCK: MASS AND CHARGE TRANSFER [Analyst, VOl. 98 Water-Throughout this series of papers, the terms water and distilled water refer to sterile, grease-free and surfactant-free water prepared in a special still,15 and having a total solids content of less than 10-l2 M, calculated as chloride.It contains dissolved carbon dioxide and oxygen, which are removed by methods described earlier.16917 Primary standard sodium carbonate solution, 0.05 M-This solution was made by direct weighing of P.V.S. sodium carbonate that had been dried at 280 "C for 2 hours. Primary standard sodium oxalate solution, 0.05 M-This solution was made by direct weighing of anhydrous sodium oxalate that had been dried at 105 "C for 2 hours. Primary standayd potassium dichi.omate solution, 0.01 6 67 M-This solution was made by direct weighing of potassium dichromate prepared by twice recrystallising AnalaR potassium dichromate from water, pulverising the crystals and drying them at 160 "C for 3 hours.Potassium permanganate solution, 0-02 Ki-Potassium permanganate was dissolved in water, the solution heated to boiling-point, simmered for 1 hour, cooled, filtered through a porosity 4 sinter and transferred into a conditioned amber Winchester bottle. A 25.00-ml aliquot of the 0-05 M sodium oxalate solution was transferred into a 250-ml conical flask containing 8.0 ml of 4.0 M sulphuric acid, the solution heated to between 80 and 90 "C and titrated with the permanganate solution to the first perceptible permanent pink colour. A blank of 8 ml of 4.0 M sulphuric acid in 50 ml of water was titrated in a similar way. In the early stages of the oxalate titration, the permanganate was added very slowly, so that the solution became colourless before the next drop of permanganate was added, until the accumulated manganese(I1) rendered the reaction fast.The standardisation was repeated before each period of use of the permanganate solution. A relative standard deviation on this and all other titrations of ,t0-04 per cent. was secured. Vanadium( V ) solutions-A solution of ammonium metavanadate was boiled with a small excess of potassium hydroxide until all the ammonia was removed. The solution was cooled and diluted to 0.1 M with water. Standardisation was effected by reducing a large aliquot of the solution with sodium sulphite and sulphuric acid, and the excess of sulphur dioxide was removed by refluxing the solution for 1 hour in a stream of nitrogen. The solution was cooled and diluted to its original volume with water.Aliquots of the vanadium(1V) solution were titrated with the standardised permanganate solution, with sodium diphenylamine- 4-sulphonate as the indicator. The 0.1 M potassium vanadate solution was used to prepare solutions of 0.05 M vanadium(V) in 2.0 M sulphuric acid, and 0-05 M vanadium(V) in 4.0 ni hydrochloric acid. Acetate bufer supporting electrolyte, pH 4-A 0.45 M solution of sodium acetate was saturated with potassium sulphate and then acidified with glacial acetic acid to pH 4, as indicated by glass and calomel electrodes and a 39A pH meter. ELECTRODE ACTIVATION PROCEDURES- Davis3** oxidised platinum electrodes chemically with a solution of silver( IT) oxide in 6.0 M nitric acid, and reduced them either potentiostatically, or by soaking them in iron(I1) sulphate solution.He obtained reproducible results if the oxidation immersion time was long enough. Ansonlg suggested that this method of oxidation could leave an adsorbed layer of silver on the electrode, which might have misled Davis to conclude that a thin platinum oxide film could accelerate oxidation - reduction processes. Iron(I1) in 1.0 M sulphuric acid was used by Anson18 for reducing platinum oxide films, and this method was found by Bishop and Riley17 t o be more reliable than electrolytic reduction in preparing platinum for the reduction of silver. Anson18 found that the oxide was relatively stable to iron(I1) in 1 . 0 ~ perchloric acid, and Kabanova has confirmed this observation.lg After many trials with diverse activation procedures, the following methods were selected for extensive testing.( a ) Chemical reduction with iron(l1)-The electrode was immersed in 0.5 M iron(I1) sulphate in 1.0 M sulphuric acid for 10 minutes at room temperature, followed by thorough washing with water. (b) Electrolytic cycling and reduction-The auxiliary electrode used in all electrolytic treatments was made of platinum. The working electrode in 1.0 M sulphuric acid was anodised a t 300 mA cm-2 for 30 s, followed by cathodisation at 300 mA cm-2 for 30 s; the anodisation and cathodisation were repeated once more, and the electrode was finally washed with distilled water.August, 19731 KINETICS AND COULOMETRIC CURRENT EFFICIENCIES. PART VII 555 (c) Chemical plus electrolytic strippiag and cathodisation-The electrode was immersed in fresh aqua regia a t 60 "C for 30 s, washed with distilled water, anodised in 11.6 M hydro- chloric acid at 208 mA cm-2 for 30 s, and again washed.Finally, the electrode was cathodised a t 230 niA cm-2 for 10 minutes in 1.0 M sulphuric acid and washed thoroughly. (d) Electrolytic reduction-The electrode was cathodised a t 300 mA cm-2 for 6 minutes in 1 . 0 ~ sulphuric acid and then washed thoroughly. ( e ) Electrolytic cycliag and oxidation-After two cycles of anodisation - cathodisation as in method ( b ) , the electrode was anodised for a third time at 300 mA cm-2 for 30 s and thoroughly washed. Unless otherwise stated, the electrodes were used directly after the last wash; any hydrogen present would be removed chemically on immersion in the electrolyte.RESULTS AND DISCUSSION First, in order to make best use of pattern theory,2O the conditional potential of the vanadium(V) - vanadium( IV) system and its dependence on hydrogen-ion concentration were determined. Then the voltammetric behaviour of vanadium(V) over a range of hydro- gen-ion concentrations was examined with respect to the diverse electrode treatments outlined above. In order to abbreviate the presentation, three conditions will be discussed, acetate buffer at pH 4.0, strongly acidic medium, 2.0 M stllphuric acid, and an intermediate state of pH 2.0 in sulphuric acid medium. Unless otherwise stated, electrodes were used for a single scan and then re-activated before further use. THE CONDITIONAL POTENTIAL OF THE VANADIUM(V) - VANADIUM(IV) SYSTEM- The zero-current equilibrium potential was found to obey the relationship, .... .. RT [VV] F [VIV] - * E,, = EL + - In ~ where EL is the conditional potential, which depends on the hydrogen-ion concentration of the medium, as shown in Fig. 1. For solutions 0.1 M or more in sulphuric acid, the acid content was determined by titration against primary standard sodium carbonate solution. 1.1 2 1 0 - I -2 -3 -4 -5 -6 Log (hydrogen-ion concentration/mol I-') Fig. 1. Variation of the conditional potential of the vanadium (V)- vanadium(1V) system with hydrogen-ion concentration (S.H.E. denotes standard hydrogen electrode). e, Values measured in sulphuric acid media; X , values measured in acetate buffer of pH 4.0; and I, range of values reported by Davis3 for sulphuric acid medium For less acidic solutions, the pH was measured potentiometrically by using glass and calomel electrodes calibrated with pH standards.The zero-current potential was independent of the method of electrode activation if sufficient time was allowed for the potential to become stabilised. Methods (b) and (c) produced electrodes that became stabilised faster than electrodes treated by other methods; but even for these two methods the stabilisation time varied, from 10 minutes to several hours, indicating very small exchange currents.556 BISHOP AND HITCHCOCK: MASS AND CHARGE TRANSFER [A.italyst, Vol. 98 VOLTAMMETRY OF VANADIUM (V)- It was quickly evident that freshly acidified vanadate solutions gave erratic results compared with aged solutions.This behaviour is caused by kinetically slow isopolymerisa- tion, probably21 the step at pH 7.0 to 6.8: Reaction PH 2 [V0413- + 2 H+ + [V,O7I4- + H20 . . .. . . 12 to 10.6 (2) 2 H4[V501,]3- + 6 H+ $ 5 V205 + 7 H,O . . .. . . 2.2 (5) V,O, + 2 H+ s 2 [VO,]+ + H20 . . .. . . (1.0 (6) 2 [V2O7l4- + 4 H+ + H2[\74013]4- + HZO . . . . 9.0 to 8-9 (3) 5 H2[V40,3]4- + 8 H+ + 4 H4[V5016]3- + H20 . . . . 7.0 to 6.8 (4) Israel and Meitesll obtained evidence from ultraviolet spectra that ammonium metavanadate dissolved in sulphuric acid slowly changed form. A recent review22 indicates that slow steps are involved in the polymerisation reactions, but that the situation is more complicated than equations (2) to (6) suggest, because the degree of polymerisation depends also on the vanad- ium concentration.To eliminate the external complication of polymerisation equilibria, all voltammetric work was carried out with solutions that had been appropriately acidified and stored for several weeks. The voltammetric behaviour of vanadium(V) in saturated potassium sulpliate solution containing sulphuric acid in the range lod5 to 2.0 M was examined; slight differences in electrode pre-treatment caused significant changes in the voltammograms. Solutions of pH about 2 gave very erratic results. In none of the experiments performed in this study was any evidence found for the reduction of vanadium below the +4 oxidation state. Anson5 and Davis3 both found chronopotentiometric evidence for the formation of vanadium( 111) at freshly reduced platinum.They also found that the electrode quickly became deactivated towards the second step at +0.25 V. In most of the runs in the present study ramp speeds of &lo0 mV min-l were used, and the time required for the electrode to deactivate towards vanadium(II1) formation was exceeded before the potential for reduction of vanadium(1V) was reached. The cause of this deactivation is not clear, but adsorption of impurities or polymeric vanadium species on the electrode seems likely. REDUCTION AT pH 4- A single reduction wave appeared for reduced electrodes [treatment (b) or (c)] in both supporting electrolytes, saturated potassium sulphate solution adjusted to pH 4 with sulphuric acid or the saturated potassium sulphate - acetate buffer.In Fig. 2, for the acetate buffer medium, curves 1 and 2 represent the background currents for reduced ( b ) and oxidised (e) electrodes, respectively. The oxidised electrode showed a maximum at about 0.7 V corresponding to reduction of the oxide film on the electrode. Addition of vanadate to a concentration of 1.09 x loA3 M gave a wave that is superimposed on curves 1 and 2 to give the composite curves 3 and 4. Chemically reduced electrodes (a) gave curves similar to 1 and 3, but the curves were not as reproducible. After curve 2 had been recorded, it was found possible to reproduce it without reactivating the electrode, so a single scan did not deactivate the electrode, and the supporting electrolyte was free from impurities that could be adsorbed.An activated electrode remained stable for 30 minutes or more in the vanadium solution, whether on open circuit or used for scanning, so the whole system could be judged to be free from readily adsorbable impurities.14 The shape of the vanadium wave at pH 4.0 is independent of the medium, but the buffer introduced a second wave, which was identified as resulting from the direct reduction of un-ionised acetic acid. 2 CH3COOH + 2e- $ 2 CH3COO- + H, . . .. - * (7) The observation of Laitinen and I l ~ l t h o f f , ~ ~ that in the presence of a large excess of sodium acetate the limiting current of the acetate wave was proportional to the total con- centration of acetic acid, was confirmed. The existence of the acetic acid wave nullified the benefit of shifting the background reactions towards more negative potentials by raising the pH.On the other hand, hydrogen ions are consumed in the reduction, and sulphuricAugust, 19731 KINETICS AND COULOMETRIC CURRENT EFFICIENCIES. PART VII I 1 557 0.9 0.6 0.3 0.0 -0.3 Electrode potential versus S.H.E./V Fig. 2. Reduction of vanadium(V) in acetate buffer saturated with potassium sulphate a t pH 4.0: 1, supporting electrolyte alone, electrode treatment (b) ; 2, supporting electrolyte alone, electrode treatment (e) ; 3, supporting electrolyte + 1.09 x M vanadium(V), electrode treat- ment ( b ) ; and 4, supporting electrolyte + 1-09 x M vanadium(V), electrode treatment ( e ) . Ramp speed - 100 mV min-1 acid at pH 4, although adequate for voltammetry, would not provide a sufficient supply unless it was supplemented at intervals during a coulometric determination.The use of a buffer solution in which the salt predominates would ameliorate the situation, but no suitable system could be found. The height of the vanadium(V) reduction wave was propor- tional to concentration over the range 0.3 to 3.5 x M. The mass-transfer rate constant was smaller in acetate buffer than in sulphuric acid, because of the higher viscosity of the buffer medium. In both media the limiting current of vanadium reduction decreased with decreasing stirring speed. 0 6 I 1 a E -.- + Lo > S .- 1 -2 0.9 0.6 0.3 0.0 Electrode potential versus S.H.E./V Fig. 3. Reduction of vanadium(V) in 2.0 M sulphuric acid: 1, 2.0 M sulphuric acid, electrode treatment (b) ; 2, 2-0 M sulphuric acid, electrode treatment (e) ; 3, 2.0 M sulphuric acid + 1.13 x M vanadium(V), electrode treatment ( b ) ; 4, 2.0 M sulphuric acid + 1-13 x 10-3 M vanadium(V), electrode treatment ( e ) ; 5, constructed by subtracting curve 2 from curve 4, to compensate for oxide reduction; and 6, recorded immediately after curve 3 was recorded, without intervening activation of the electrode.Ramp speed - 100 mV min-1558 BISHOP AND HITCHCOCK: MASS AND CHARGE TRANSFER [Analyst, VOl. 98 REDUCTKON IN 2.0 M SULPHURIC ACID- Voltammetric curves in this medium (Fig. 3) are more complicated than those at pH 4, but less complex and more reproducible than those at pH 2. The reduction wave showed a limiting current region that varied slightly in height but very greatly in the rising portion of the wave with the method of electrode activation. The chemical treatment, (a), gave much less reproducible curves than the electrolytic treatments and was therefore abandoned.Treatments (b) and (c) gave very similar voltammograms but method (b) gave rather more reproducible results, although even then a 5 per cent. difference had to be accepted as being satisfactory. Scan 1 of the background with a reduced [treatment ( b ) ] electrode showed that the electrolyte and electrode were clean. Scan 2 is of an oxidised electrode [treatment (e)] and shows the oxide reduction peak. When the oxidised electrode is used to reduce vana- dium(V), the oxide reduction peak is superimposed on the vanadium wave, as in curve 4. By subtracting curve 2 from curve 4, curve 5 was obtained and showed that the rate of reduction of vanadium(V) remained very slow until some of the oxide on the platinum surface had been reduced, but when about 80 per cent.of the oxide had been reduced, the electrode behaved like a pre-reduced electrode. The observation that the rate of reduction of vanadium(V) decreased at an oxidised platinum surface is in agreement with the findings of Anson and King.5 It is not, however, easy to explain why the electrode in this experiment behaves as if it were fully reduced, when curve 2 indicates that oxide remains on the electrode. The curves in Figs. 2 and 3 were drawn at the same ramp speed, yet the two oxide reduction curves differ; that in Fig. 3 is less sharp and shows tailing on the cathodic side: the tailing could be diminished by using a slower ramp speed.Curve 3 in Fig. 3 shows that reduction of vanadium(V) at a pre-reduced electrode is not simple: a minimum appears at about 0.6 V. This behaviour occurred whenever the hydrogen-ion concentration of the solution was such that vanadium(V) reduction could be observed at potentials more positive than 0-6 V. Although the conditional potential of the vanadium(V) - vanadium(1V) system could be changed by 0.4 V or more by changing the hydrogen-ion concentration, there was very little change in the potential at which the minimum occurred, which is close to the potential of the platinum oxide reduction maximum, so that it seemed possible that the two might be related. The chemical effect of a 10-3 M solution of vanadium(V) in 2.0 M sulphuric acid on a reduced electrode prepared by method (b) was investigated by immersing the fresh electrode in this solution for 2 minutes, washing it well with distilled water, transferring it into a pure 2.0 M sulphuric acid solution and making a cathodic scan.The scan showed a small maximum at 0.65 to 0.70 V, thus indicating light oxidation of the electrode surface. Repetition of this procedure with 2.0 M sulphuric acid alone showed that the oxidation occurred chemically in the vanadium(V) solution and not during the washing or transfer. Soaking the electrode for 15 s in the vanadium(V) solution produced a detectable amount of oxide. Although a fully reduced electrode can be immersed in the vanadium(V) solution for scanning, the electrode surface was lightly oxidised before the scan was started.The electrode had to be carefully positioned, as did the Luggin capillary, then connections had to be made to the ramp generator and the circuit tested. This process took 15 to 30 s to complete. The chemical oxidation of the working electrode, reduction of the oxide at about 0-6 V and the minimum in the vanadium(V) reduction wave at pre-reduced electrodes [methods (b) and (c)] appear to suggest that light oxidation of the electrode does facilitate the vanadium reduction as compared with a fully reduced electrode. If this be so, then vanadium(V) in 2.0 M sulphuric acid is reducible at three different rates, depending on the amount of oxide on the electrode surface. A heavy oxide film [treatment (e)] must, on the evidence of curve 5 in Fig.3, permit only very slow reduction of vanadium(V). The light oxidation by chemical attack by vanadium(V) permits a relatively fast reduction, while a fully reduced electrode (formed when the potential becomes negative to 0.6 V) must reduce vanadium(V) at an intermediate rate. This effect was, in part, confirmed by lowering the zero-current potential of the vanadium solution by addition of vanadium(IV), so that chemical oxidation did not occur. Curve 1 in Fig. 4 shows no minimum at 0.6 V in the reduction wave, and both k and cc are obviously decreased. Curve 3 shows the normal behaviour in the initial absence of vanadium(1V) under otherwise the same conditions. Curve 2 is the immediate reverse scan after curve 1 and shows the large hysteresis.August, 19731 KINETICS AND COULOMETRIC CURRENT EFFICIENCIES.PART VII 0.04 I 1 I CI c 6 0.03 559. 1 I 1 I 1 1 1 1 1.2 1.0 0.8 0.6 0.4 0.2 0;O -0.2 Electrode poten t i a I versus S. H. E ./V Fig. 4. Reduction of vanadium(V) in 0-1 M sulphuric acid: 1, 0.1 M sulphuric acid + 1.0 x M vanadium(V) + 1.0 x M vanadium(IV), electrode treatment ( b ) ; 2, reverse scan at + 100 mV min-1 immediately after recording curve 1; and 3, 0-1 M sulphuric acid + 1.0 x 10-4 M vanadium(V), electrode treatment (b). Ramp speed -100 mV min-l POSSIBLE ADSORPTION EFFECTS IN THE REDUCTION OF VANADIUM(V) IN 2.0 M SULPHURIC ACID- The hysteresis just mentioned is not confined to solutions containing vanadium( IV). Reverse scans are not shown in Fig. 3 in order to avoid confusion; reverse scans in the context have little significance, but such scans were performed and also showed hysteresis, It appeared possible that a known trace impurity in the sulphuric acid may become adsorbed on the elect rode.Aristar sulphuric acid diluted to 10-0 BI was cleaned by electrosorption, as will be des- cribed in a later paper, and the solution used to prepare the supporting electrolyte. Back- ground and vanadium(V) reduction scans showed no significant difference from those in Fig. 3, so adsorption of impurities could be eliminated. Nevertheless, it is possible that adsorption of vanadium species could be responsible for hysteresis and for the difference between scan 3 in Fig. 3 with the electrode reduced by method (b) and scan 6 performed immediately afterwards with the same electrode and without re-treatment.This conclusion was supported by an experiment in which an electrode used to record a curve similar to scan 3 in Fig. 3 was subjected to treatment (d) and then scanned in the same solution. The result was a curve very similar to curve 6 in Fig. 3, and showed that simple cathodisation was not so effective as treatments (b) and (c) in producing an active electrode. The difference cannot be due to any form of platinum - oxygen structure produced during pre-treatment, because treatment (c) does not produce an oxidised surface during the anodic treatment, but merely strips platinum as the soluble hexachloroplatinic(1V) acid. The difference between treatments (b) and (c) and treatment (d) lies in the ability of ( b ) and (c) to clean the electrode by anodic stripping.Deactivation of electrodes prepared by methods (b) and (c) cannot be attributed to reduction products of the electrode reaction, because a freshly activated electrode does become deactivated on standing at zero current in the vanadium(V) solution. It is possible that the chemically produced film may contain vanadium species, perhaps in a mixture of oxidation states. In 2.0 M sulphuric acid a kinetically slower curve than scan 6 in Fig. 3 could not be obtained either by allowing the electrode to stand in the solution or by continually scanning the reduction wave, which showed conclusively that on deactivation the electrode quickly attains a stable and reproducible form. REDUCTION I N SULPHURIC ACID AT pH 2- The behaviour of vanadium(V) at the variously treated electrodes in sulphuric acid solutions of 2.0 M down to 0-1 M concentration was found to be similar to that discussed for 2.0 M acid, but with further decrease in acid concentration changes occurred in the voltam- mograms.Electrodes treated by methods (b) and (c) again showed a minimum, as in 2.0 M sulphuric acid, but at pH 2 the height of the wave before the minimum was not reproducible, and could In illustration the behaviour at pH 2 will be considered.560 BISHOP AND HITCHCOCK: MASS AND CHARGE TRANSFER [Analyst, Vol. 98 change by 50 per cent. from one experiment to the next, but was always lower than the height of the main wave formed at potentials cathodic to the minimum. The variability of the pre-wave is probably due to small differences in the amount of chemical oxidation suffered by the platinum surface before the scan was recorded. As the hydrogen-ion concentration decreases, the oxidising power of the solution decreases as shown by the conditional potentials in Fig.1. It has been ~ h ~ ~ n ~ ~ p ~ ~ that the amount of oxide formed on platinum depends on the time of contact with the oxidising solution and the potential of the solution. The amount of oxide formed at pH 2 seems more time dependent than in 2.0 M sulphuric acid. Attempts to verify this observation by coulometric stripping of electrodes that had been soaked for different periods in vanadium solutions were frustrated by the errors introduced by charging currents. An electrode activated by method (b) gave as its first and second scans curves 1 and 2 in Fig.5. The deactivation after the first scan was greater than that for a single scan in 2.0 M sulphuric acid. Curve 3 shows the effect of leaving a similar electrode in the vanadium solution for 80 minutes at zero-current potential before starting the scan. In 2-0 M sulphuric acid a single scan gave full deactivation of the electrode; at pH 2.0 the deactivation was greater, and the act of scanning accelerated it. It seems likely from equation (5) that at pH 2.0 colloidal vanadium(V) oxide could form and stick on the electrode, where it could be reduced to an insoluble species such as an oxovanadium(1V) polyvanadate(V), which would interfere with mass transfer. 09 0.6 0.3 00 Electrode potential versus S.H.E./V Fig.5. Reduction of vanadium(V) in sulphuric acid a t pH 2.0. All solutions 1.6 x M vanadium(V) in 5 x Methods used to activate the electrodes : 1, method (b) ; 2, no reactivation after curve 1, used immediately; 3, method ( b ) , then left a t zero current in the vanadium solution for 80 minutes before being recorded; 4, method ( d ) ; and 5, method (e). Ramp speed -85 mV min-l M sulphuric acid adjusted to pH 2.0. The effect of the cathodic activation (d) applied to a used electrode is shown in curve 4 i n Fig. 5, and is similar to the behaviour in 2.0 M sulphuric acid. Unlike the latter behaviour, however, an electrode having once been activated and used could be reactivated by method (d), so that the following scan showed a faster reaction than it would have shown if no additional activation had been applied.Treatment (e) produced the same shape of curve as in 2 . 0 ~ sulphuric acid, with a potential shift of the leading edge corresponding to the shift in with the change in hydrogen-ion concentration. For an activated electrode, the true limiting current at 0-1 V was proportional to the concentration of vanadium(V) in the solution. While the height of the current maximum at 0.75 V increases with increasing vanadium(V) con- centration, the relationship is not proportional.August, 19731 KINETICS AND COULOMETRIC CURRENT EFFICIENCIES. PART VII 561 As the pH of the vanadium(V) solution in sulphuric acid increased from 2 to 3 the shape of the voltammograms remained similar to those at pH 2, except for the expected shift to more negative potentials.Furthermore, the electrodes showed less deactivation by the vanadium(V) solutions. When the pH increased to 3.5, the platinum oxide reduction maximum and the vanadium reduction wave no longer coincided and the curves produced resembled those in Fig. 2. At about this hydrogen-ion concentration, the oxidation potential of the vanadium solution dropped below that at which platinum can be extensively oxidised, thus simplifying the voltammograms. The decrease in deactivation of the electrode as the pH increased from 2 to 4 can be explained by the reversal of reaction (5) to produce soluble species such as the tetrahydrogenpentavanadate (V) ion. VOLTAMMETRY OF VANADIUM(IV) SOLUTIONS- The high conditional potential and the low charge-transfer speed of the vanadium(V) - vanadium(1V) process forced the oxidation of vanadium(1V) to such a high anodic potential that no limiting current region could be isolated: the wave merged with the solvent oxidation wave.A typical scan in acetate buffer medium is shown in Fig. 6, and as the hydrogen-ion concentration increased in sulphuric acid media the merger of vanadium and solvent waves became more complete until in 2.0 M sulphuric acid the presence of a vanadium wave could scarcely be detected. Davis26 found that a chronopotentiometric transition time for vana- dium(1V) in 1 . 0 ~ sulphuric acid could not be measured on account of the simultaneous oxidation of water. Media with pH higher than 4 have not been examined, but it is possible that the change of E; with pH would be greater than the shift in solvent oxidation potential and a separate wave might be attainable at pH 6.The curve in Fig. 6 was obtained with cathodised electrodes [treatment ( b ) , (c) or (41, but the working potential is such that an anodic film must form on the electrode. A heavily oxidised electrode [treatment (e)] was found to give an even slower oxidation, and a rather erratic performance. t 1-5 1.0 0.5 0.0 Electrode potential versus S.H.E./V Fig. 6. Anodic oxidation of vana- dium(1V). Electrode activated by method (!), acetate buffer saturated with potas- sium sulphate and adjusted to pH 4.0, 2.78 x M vanadium(IV) and 0-21 x Ramp speed + 100 mV rnin-l M vanadium(V). CONCLUSIONS The voltammetric behaviour of the vanadium(V) - vanadium( IV) system at platinum electrodes is highly dependent on the composition of the supporting electrolyte and hydrogen- ion concentration, and on the pre-treatment of the electrodes. It is further complicated by participation of the electrode surf ace in chemical and electrochemical reactions, and by562 BISHOP AND HITCHCOCK direct reduction of the weak acid in buffer media.The values of the mass and charge transfer kinetic parameters will be discussed in a later paper after the behaviour in the presence of other transition-metal compounds has been described. There is no evidence in the voltam- metry of pure vanadium(V) and vanadium(1V) solutions that the reaction is other than a single-step one-electron process, although there must at least be a following chemical step, either reaction of VO, with hydrogen ion to form V02+ or reaction of V03+ with solvent to give VO,+.Isopolymers must be involved at higher pH values, and what appears to be a one electron - one vanadium atom process may be a five electron - one tetrahydrogen- pentavanadate(V) ion reaction, certainly by an electron tunnelling process, followed by decomposition or rearrangement of the polymer. A one electron - one tetrahydrogenpenta- vanadate(V) ion reaction is unlikely in view of the similar mass-transfer rates in 2.0 M sulphuric acid and in acetate buffer. Although the reproducibility cannot be considered excellent, the voltammograms of the reduction of vanadium(V) in 2.0 M sulphuric acid (Fig. 3) and in acetate buffer at pH 4 (Fig.2) show sufficiently well defined limiting current regions to permit the choice of control potentials for potentiostatic reduction of high efficiency. Vanadium(1V) cannot be oxidised without severe loss of current efficiency, nor reduced to vanadium(II1) at platinum electrodes. We express our sincere gratitude to Imperial Chemical Industries Limited for a research grant 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. coveiing a period of 3 years. REFERENCES Lingane, J. J., J . Amer. Chew. Soc., 1945, 67, 182. Miller, F. J., and Zittel, H. E., J . EEectroanalyt. Chem., 1964, 7, 116. Davis, D. G., Analyt. Chem., 1959, 31, 1461. -, J - Electroanalyt. Chem., 1960, 1, 73. Anson, F. C., and King, D. M., Analyt. Chem., 1962, 34, 362. James, S. D., J . Electrochem. SOG., 1967, 114, 1113. Meier, D. J., Myers, R. J.. and Swift, E. H., J . Amer. Chem. SOC., 1949, 71, 2340. Furman, N. H., Reilley, C. N., and Cooke, W. D., Analyt. Chem., 1951, 23, 1665. Kennedy, J. H., and Lingane, J. J., Ibid., 1958, 18, 240. Lingane, J. J., Analytica Chim. Acta, 1956, 15, 465. Israel, Y., and Meites, L., J . Electvoanalyt. Chem., 1964, 8, 99. Davis, D. G., Ibid., 1960, 1, 73. Bishop, E., and Hitchcock, P. H., Analyst, 1973, 98, 465. Bishop, E., and Sutton, J. R. B., Analytica Chim. Acta, 1960, 22, 690. Bishop, E., and Riley, M., Analyst, 1973, 98, 305. -- , Ibid., 1973, 98, 416. Anson, F. C., Analyt. Chem., 1961, 33, 934. Kabanova, 0. L., Russ. J . Phys. Chem., 1961, 1219. Bishop, E., Analyst, 1972, 97, 761. Remy, H., “Treatise on Inorganic Chemistry,” Elsevier Publishing Company, Amsterdam, 1956, Pope, M, T., and Dale, B. W., Q. Bev. Chem. Soc., 1968, 22, 527. Laitinen, H. A., and Kolthoff, I. M., J . Phys. Chem., 1941, 45, 1061. Kolthoff, I. M., and Tanaka, N., Analyt. Chem., 1954, 26, 632. Nicholson, R. S., and Shain, I., Ibid., 1965, 37, 178. Davis, D. G., J . Electvoanalyt. Chem., 1960, 1, 73. NOTE-References 13, 14 and 20 are to Parts V, VI and 111, respectively, of this series. I , Ibid., 1973, 98, 475. -- Volume 11, p. 100. Received January 24th, 1973 Accepted March 26th, 1973

ISSN:0003-2654    

DOI:10.1039/AN9739800553

出版商:RSC    

年代:1973

数据来源: RSC

摘要:

Amalyst, August, 1973, Vol. 98, $9. 563-571 563 Mass and Charge Transfer Kinetics and Coulometric Current Efficiencies Part VIII.* Single-scan Voltammetry of Vanadium(V) - Vanadium( IV) in the Presence of Chromium, Manganese and Iron, and the Kinetic Parameters of the Vanadium System, at Platinum Electrodes Pre-treated by Five Methodsf. BY E. BISHOP AND P. H. HITCHCOCKS (Chemistry Department, University of Exetey, Stocker Road, Exeter, EX4 4QD) Continuing the earlier examination of the vanadium system alone, under various conditions and with various electrode pre-treatments, the effect of neighbouring steel-forming d-block elements has been investigated. Chro- mium(V1) at pH 4.0 suppresses the vanadium(V) reduction wave, and the degree of suppression is quantitatively proportional to the chromium(V1) concentration.Activated electrodes are deactivated by dipping them in a chromium(V1) solution, and remain so even when well washed thereafter, so that chromium(V1) as well as chromium(II1) is adsorbed strongly on platinum. In 2.0 M sulphuric acid, chromium(V1) and vanadium(V) are reduced at the same rate. Manganese(VI1) in acetate buffer gives a fast, well separated wave, but the separation is not as good in 2-0 M sulphuric acid ; slowing the vanadium(V) reduction by using an oxidised electrode effects no improvement : the manganese wave is similarly affected. Addition of chromium(V1) to the manganese - vanadium mixture a t pH 4 suppresses the manganese wave only slightly, even when the vanadium wave is com- pletely suppressed. In 2.0 M sulphuric acid, the manganese wave is un- distorted and chromium and vanadium are simultaneously reduced.Iron (111) in 2.0 M sulphuric acid does not interfere, but the separation of the vanadium and iron waves is not good. Iron(I1) can, however, act as a potentiostatic intermediate. The kinetic parameters of the vanadium system are repro- ducible in acetate buffer, but only when the electrode is fouled in 2.0 M sulphuric acid. Pattern theory and diffusion-corrected Lewartowicz methods give results that agree. The charge-transfer kinetic parameters are shown to be potential dependent in acidic media. The results are compared with those in earlier reports. The generation current efficiency for vanadium(1V) in acetate buffer was computed. IN a previous paper,l the influence of hydrogen-ion concentration on the conditional potential of the vanadium(V) - vanadium(1V) system was examined, and the voltammetry of the system in sulphuric acid at concentrations ranging from 5 x to 2-0 M, and in potassium sulphate - acetate buffer at pH 4-0, was described.Platinum electrodes treated by five different methods, (a) chemically reduced with iron(I1) in sulphuric acid, (6) cyclically anodised and cathodised and reduced, (c) chemically and electrochemically stripped and electrochemically reduced, (d) cathodically reduced and (e) anodically oxidised, were investigated. Method (a) was of little use, while method (e) invariably gave the slowest charge transfer. Method (d) was moderately effective, but the preferred method was (b) because the anodic stripping appeared to remove some adsorbate from the electrode surface.These experiments related t o pure solutions of vanadium in pure supporting electrolyte. The objective was to measure the electrode kinetic parameters with a view to selecting control potentials for potentiostatic coulometry, and to determine the current efficiency under various conditions. With the further objective of application to the determination of vanadium in real samples, such as alloy steels, the influence of neighbouring d-block elements was investigated in order to discover whether they offered any interference, and also whether they could be determined * For Part VII of this series, see p. 553. t Presented a t the Second SAC Conference, Nottingham, 1968. Present address: Ever Ready Co.(G.B.) Ltd., Central Research Laboratory, St. Ann’s Road, London, N15 3TJ. @ SAC and the authors.564 sequentially in a mixture. behaved as expected. BISHOP AND HITCHCOCK: MASS AND CHARGE TRANSFER [Analyst, Vol. 98 Chromium and manganese gave surprising results, but iron EXPERIMENTAL Apparatus and procedures have been describedY2 the electrode pre-treatments are given in detail and the reagents and supporting electrolytes and their standardisation or treatment are set out in Part VII.1 Of the latter, the 2.0 M sulphuric acid and the saturated potassium sulphate - acetate buffer adjusted to pH 4.0 have been chosen for this further study, because the behaviour of the system is least complicated when the vanadium is present entirely as the cations, V02+ and VO,+, or as the tetrahydrogenpentavanadate ion, H,(V501s)3-.RESULTS AND DISCUSSION EFFECT OF CHROMIUM(VI)- Hypothetically (because the charge-transfer process is extremely slow and the exchange current is minute) the separation of the conditional potential of the chromium(V1) - chromium(II1) system from that of vanadium(V) - vanadium(1V) is adequate to permit the sequential determination of chromium(V1) and vanadium(V) . The vanadium reduction is kinetically slow,l and the chromium reduction much slower, so perhaps the sequence vana- dium(V) then chromium(V1) could be realised in practice instead. In the presence of iron(III), the faster reduction of iron(II1) would produce iron(II), which would behave as a potentio- static intermediate and so restore the order to chromium(V1) first and then vanadium(V).0.4 - 0.3 - 0.2 - 0.1 - 0.0 - - 0.9 0.6 0.3 0.0 -0.3 Electrode potential versus S.H.E./V Fig. 1. Effect of chromium(V1) on the reduction of vana- dium(V) in saturated potassium sulphate - acetate buffer at pH 4.0. Before each curve was recorded, the electrode was activated by method ( b ) : 1, supporting electrolyte alone; and 2, supporting electrolyte + 1.5 x M vanadium(V). Portions of a chromium(V1) solution were then added to give the following [Vv] to [CrvI] ratios: 3, 92; 4, 46; 5, 31; 6, 19; and 7, 14. Ramp speed - 1.0 mV s-l However, experimentally, in the pH 4 buffer supporting electrolyte the addition of small amounts of chromium(V1) to a vanadium solution caused the vanadium(V) limiting current at freshly activated reduced electrodes [methods (b) and (c)] to decrease.This remarkable behaviour is shown in Fig. 1. Over a limited range of chromium(V1) concentrations, the decrease in the vanadium limiting current is directly proportional to the chromium(V1) content of the solution. In such solutions, if an electrode was re-used without intervening reactivation the vanadium limiting current progressively decreased until the vanadium(V) wave disappeared entirely. For the curves in Fig. 1, the vanadium concentration, although small, is much larger than the chromium concentration ; when equal concentrations of vanadium(V) and chromium(V1) were present no reduction wave appeared for either species. The reproducibility is such that, for the particular conditions and concentrations used,August, 19731 KINETICS AND COULOMETRIC CURRENT EFFICIENCIES.PART VIII 565 chromium(V1) in the concentration range to 1 0 - 4 ~ could be determined to within 5 5 per cent. by the suppressive effect. The possible development of the method to trace levels is to be pursued. This behaviour cannot be ascribed to formation of a conventional platinum oxide film, because no maximum was detected at 0.6 V, and all the scans are negative to this potential. Kolthoff and El Din3 claimed that a solution of chromium(V1) in mineral acids to 1W2 RI in hydrogen-ion produced a monolayer of chromium( 111) hydroxide on a platinum cathode used in the solution. The monolayer of, presumably, solvated chromium oxohydroxide had the power of suppressing some electron-transfer reactions, but no report was made on the behaviour of vanadium.Kolthoff and El Din did not examine the process at pH 4, but the present work indicates that the film thickness or coverage is dependent both on the chromiuni(V1) concentration and on the number of scans performed with the electrode. It proved possible to deactivate a freshly activated electrode [method (b), (c) or ( d ) ] simply by dipping it into a 1.67 x M sulphuric acid for 5 minutes. When the electrode was removed, thoroughly washed with water and used to scan a vanadium(V) solution in acetate buffer, the limiting current was 10 per cent. less than that for a similarly activated electrode that was washed but not exposed to the chromium solution. Chromium(V1) can therefore be directly adsorbed on the surface of an electrode, and when a cathodic current is applied in a weakly acidic medium [in which the chromium(II1) hydroxide is presuniably insoluble] a layer of chromium( 111) hydroxide forms on the electrode surface.It is not, however, directly proved that the chromium is reduced completely or even reduced at all: formation of chromium(II1) chromate(VI), which is sparingly soluble, is not unlikely. In the 2.0 M sulphuric acid supporting electrolyte both species are reduced at the same rate ; the chromium(V1) wave coincides with the vanadium(V) reduction wave, as Davis4 has found. Kolthoff and El Din3 found that, in mineral acids more concentrated than 0.1 M, chromium(V1) did give a reduction wave at platinum cathodes, and identified the product as chromium(III), but do not mention the current efficiency. They concluded that this finding supported their contention that the film formed on a platinum cathode at lower hydrogen-ion concentrations from chromium (VI) was the hydroxide, but this interpret ation is not unequivocal.Specific adsorption of chromate or yolychromate ions on the electrode surface, and physical blockage of the surface, without reduction, does not seem unlikely. EFFECT OF MANGANESE(VII)- Scanning a mixture of manganese(VI1) and vanadium(V) in the acetate buffer supporting electrolyte at pH 4 gave a manganese wave with a half-wave potential of +0.962 V, super- imposed on the usual vanadium(V) wave as shown in Fig. 2. The manganese wave is sharp and well defined, the reaction being clearly fast, and is well separated from the vanadium wave.The limiting currents of the individual waves were found to be proportional to the concentrations of manganese(VI1) and vanadium(V) . In the 2.0 M sulphuric acid electrolyte, the vanadium(V) and manganese(VI1) waves were additive, but the separation, as shown in Fig. 3, was not good. As the hydrogen-ion concentration increased, both waves moved in the anodic direction, but the vanadium wave moved considerably more than the manganese wave, as would be expected from a knowledge of the homogeneous reaction mechanisms. Although the vanadium(V) reduction could be slowed down by using an oxidised electrode [treatment (e)] the manganese(VI1) reduction was also slowed down, and separation of the waves was not improved. When separately reduced, the product from manganese(VI1) is manganese(II), and scans of mixtures gave no evidence to the contrary.The mechanism was not fully revealed until the quantitative potentiostatic determinations, to be reported later, were carried out. In the presence of vanadium(V), manganese(VI1) is reduced to manganese(III), and not manganese(I1). Further reduction gives a combined wave representing reduction of man- ganese(II1) and vanadium(V). COMBINED EFFECT OF CHROMIUM(VI) AND MANGANESE(VII)- Addition of chromium(V1) to a mixture of vanadium(V) and manganese(VI1) in the buffer electrolyte of pH 4.0 suppressed the vanadium wave in exactly the same manner as in Fig. 1, but only slight suppression of the manganese(VI1) wave occurred.Addition of M solution of chrornium(V1) in566 0.9 0.6 0.3 0.0 Electrode potential versus S.H.E./V [Analyst, VOl. 98 Fig. 2. Reduction of vanadium(V) and manganese(VI1) in saturated potassium sulphate - acetate buffer a t pH 4.0: 1, 1.1 x M vanadium(V), electrode treatment (c); and 2, 1-1 x 10-3 M vanadium(V) + 0.13 x 10-3 M manganese(VII), electrode treatment (c) . Platinum cathode, scan speed - 80 mV min-1 sufficient chromium( VI) completely to suppress the vanadium wave decreased the rnan- ganese(VI1) limiting current by only 8 per cent. Kolthoff and El Din3 reported that the reduction of manganese(VI1) in M mineral acid was suppressed to the extent of 60 per cent. when a platinum cathode that was completely filmed with chromium(II1) hydroxide was used.They did not extend their measurements to pH 4.0, but the difference is rather striking. In the 2 . 0 ~ sulphuric acid supporting electrolyte a mixture of vanadium(V), man- ganese(VI1) and chromium(V1) behaved as can be predicted by the foregoing results. The 1.45 1.25 1-05 0.85 0.65 0-45 0.25 0.05 -0.15 Electrode potential versus S. H. E ./V Fig. 3. Reduction of vanadium(V) and manganese(VI1) in 2.0 M sulphuric acid: 1, 0.2 x M manganese(VII), electrode treatment (b) ; 2, 0.2 x 10-3 M manganese(VII), electrode treatment ( e ) ; 3, 1.13 x M vanadium(V), electrode treatment (b) ; and 4, 1.13 x M vanadium(V), electrode treatment ( e ) . Ramp speed - 100 mV min-lAugust, 19731 IUNETICS AND COULOMETRIC CURRENT EFFICIENCIES. PART VIII 567 manganese reduction wave was undistorted, and the chromium(V1) and vanadium(V) waves were superimposed one on the other.EFFECT OF IRON(III)- The behaviour of iron(II1) in acetate buffer was not examined, but in 2 . 0 ~ sulphuric acid a well defined reduction wave of half-wave potential 0-647 V was obtained, and the electrode process was clearly moderately fast. Fig. 4 shows the scans of mixtures of iron(II1) and vanadium(V) at electrodes treated by methods (b) and (d). The benefit of using the anodic stripping pre-treatment instead of the simple reduction1 is again apparent, but the separation of the two waves is not good in this medium. 0.9 0.6 0.3 0.0 Electrode potential versus S.H.E./V Fig. 4. Reduction of mixtures of vanadium(V) and iron(II1) in 2.0 M sulphuric acid [1-13 x M vanadium(V) + 1.03 x M iron(III)] : 1, electrode activation method (b) ; and 2, electrode activation method (d).Scan speed - 100 mV min-1 KINETIC PARAMETERS OF THE VANADIUM ELECTRODE REACTIONS- Redztction of va.nadi.um( V)-The work on pure vanadium solutions1 showed that the most easily reproducible scans were obtained in the potassium sulphate - acetate buffer electrolyte at pH 4.0, and the method of pre-treatment of electrodes for use in this medium is less critical because there is no chemical oxidation of the electrode surface by the vanadium(V). The next most reproducible scans were obtained in sulphuric acid at concentrations of 0-1 to 2 . 0 ~ , in which three reproducible conditions of the electrode surface could be defined as follows- (i) a reduced electrode pre-treated by method (b) or (c); (ii) an oxidised electrode pre-treated by method (e); and (iii) a reduced electrode, with adsorption, pre-treated by method ( d ) ; this condition is stable and is attained by electrodes treated by other methods after a single scan.Values for K,,,,, k and a were readily obtained from the vanadium(V) reduction waves in the acetate medium at pH 4.0, by using pattern theory5 or Lewartowicz’s diffusion-corrected method (but not his dubious second linearisation method) .2 The voltammetric scans in 0.1 to 2.0 M sulphuric acid as the supporting electrolyte (Fig. 3, Part VIP) will not yield a single set of charge-transfer parameters, because k and cc change considerably with change in working electrode potential or current, which is immediately revealed by using sequential points in the pattern theory equations, and is also shown by the curvature of the Lewartowicz plots.Such behaviour emphasises that not one, but three reactions are in progress: reduction of568 BISHOP AND HITCHCOCK: MASS AND CHARGE TRANSFER [Ana/$St, VOl. 98 vanadium(V) , reduction of platinum oxide and a potential-dependent adsorption process. The last two reactions change the nature and activity of the platinum surface and therefore the charge-transfer parameters. Empirical equations, from statistical analysis of the results from pattern theory, can be constructed, but would apply only to a single scan: the experi- mental reproducibility of the curves for sulphuric acid media is only about 5 per cent., whereas pattern theory has an accuracy of about 0.01 per cent.in the context, so that the empirical equation for one curve would be invalid for the next. Values of the mass and charge transfer parameters for the reduction of vanadium(V) under a variety of conditions are given in Table I. When the parameters are potential dependent the range of variation is given rather than a mean value, which could be misleading. Also included in Table I are the values reported by Davis.6 These values are academic and are useless in the real coulometric context. They were obtained in mixtures of van- adium(V) and vanadium(1V) in 1.0 M sulphuric acid at very low currents that did not exceed 1 per cent. of the limiting currents. The relatively small potential range covered would be unlikely to change the electrode surface condition much from its state at zero current, and so Davis was able to obtain “linear” Tafel plots that intersected at the zero-current potential.For the same reasons, Davis’ values for a and /3 summed to 1.00 & 0.06, which is certainly not valid in the wide-ranging potentials of coulometry. Fig. 5 shows three voltammetric curves computed by VOLTAMMETRY 9,’ by using Davis’ values for K and cc and assuming no alteration with potential. The difference between these curves and, for example, curve 6 of Fig. 3, Part VII, is very large indeed. Davis’ methods of electrode activation were different from those used in the present work, except for the iron(I1) treatment ( a ) , which in the present work gave a curve very similar to curve 6 of Fig.3, Part VII. The differences between the 0.5 0.4 N I E 0 03 a E 2 0.2 2 \ + > .- U t: 4 4 3 0.1 0.0 I I I 1 1 1.0 0.5 0.0 Electrode potential versus S. H. E./V Fig. 5 . Voltammograms computed from Davis’ kinetic values. Mixed vanadium(V) - vanadium(1V) in 1.0 M sulphuric acid, Davis’ values6 converted to litre units- E’,/V k / l cm-2 s-1 a 1 1.053 4.06 x 0.52 2 1.057 3.78 x 0.74 3 1.072 1.85 x 0.67 Other parameters used were: 6x, 10-3cm; k ~ , 7.72 x lcm-2 s-l; MH, 0 - 5 ; [ V ~ B , M ; WI933, 0; n, 1; km,,, ox, 3.1 x 1 S-l; and kmss, red, 3.1 X lo-’ 1 Cm-’ S1TABLE I VANADIUM CONCENTRATION OF 0.13 TO 1.9 x 10-3 M MASS AND CHARGE TRANSFER RATE PARAMETERS FOR THE VANADIUM(V) - VANADIUM(IV) REACTION AT PLATINUM ELECTRODES AT A Medium Reaction Acetate buffer, VV + e - VIV pH 4.0 Acetate buffer, VIV - VV + e§ 2-0 M H2SO4 VV$ e-VV pH 4-0 1.0 M HSSO, VV+ e-VIV ( Daviss) 2-0 M H2S04 Background cathodic Acetate buffer, Background pH 4.0 cathodic Electrode pre-treatment* or used electrode (4, (4, (4, (4 k/l cm-a s-l (7.1 - 9.3) x lo-’ 2.5 x 10-9f ** 4 x 1 0 q t 1.3 x 10-71:: 2 x lO-’tt 3.5 x l O - S i $ 2.1 x lO--’tt 3.3 x 10-8;: 4.06 X 3.78 x 1-85 x 7.72 x 1.62 x a 0.25 to 0.29 - ** 0.92 0.62 0.31 0.22 0.27 0.22 0.52 0.74 0.67 0.5 0-5 kmaes ox?/ 1 cm-2 s-1 (2.15 - 2.2) X lo-’ ** 3.1 x lo-‘ 3.1 x lo-‘ 3.1 x low6 3.1 x lo-‘ 3.1 x 3.1 x lo-‘ 9 x 10-4 9 x 10-4 * Treatments ( b ) , (G), (d) and (e) are mentioned in the introduction and are described in detail in Part V1I.l Davis’ activation proceduress : X, Oxidation by silver(I1) oxide in 6.0 M nitric acid for a “sufficient” time, then reduction in 1.0 M sulphuric acid a t -0.25 V.Y. Oxidation by silver(I1) oxide in 6.0 M nitric acid for a “sufficient” time. 2, Oxidation by silver(I1) oxide in 6.0 M nitric acid for a “sufficient” time, then reduction in iron(I1) solution in 1.0 M sulphuric acid. cm. t Rmsss OX = Dv6/8%; kmsss red = D v ~ / ~ x ; “apparent” value 8% = 1.0 x $ Results approximate on account of distortion by background wave. fj Positive-going ramp. All other scans negative-going. ** Wave too distorted by background wave to give even approximate values. t t For the low current portion of the wave, 5 to 20 per cent. of the limiting current. $$ For the high current portion of the wave, 80 to 90 per cent. of the limiting current. $3 Scan performed immediately after a previous run without reactivating the electrodes, Electrode originally activated by treatment (b) ..57 0 BISHOP AND HITCHCOCK: MASS AND CHARGE TRANSFER [Afia~?JEt, VOl.98 present and Davis’ results cannot be accounted for by differences of electrode treatment, or of the acidity of the medium, and support the finding that the charge-transfer parameters are highly dependent on the potentials at which they are measured. The oxidation wave of vanadium(1V) was always too close to the wave for the oxidation ‘of water, so that the values of k and cc [= (1 - /3)] cannot be regarded as more than an approxi- mate estimate. Considering the very large difference in potential between the waves for oxidation of vanadium(1V) and reduction of vanadium(V), it is not surprising that considerable changes in the nature of the electrode surface pertain and that the difference in rate constants for the two processes is relatively small in the context.‘COMPUTATION OF CURRENT EFFICIENCIES FROM EXPERIMENTALLY DETERMINED PARAMETERS- Because k and cc change with potential and current in sulphuric acid, the simple computa- tion of generation efficiency for the production of vanadium(1V) as an intermediate is not possible without error when using the fully developed VOLTAMMETRY 9 G/P7 program, without inserting an ecological matrix of parameters, This procedure is now possible with the latest satellite programs to VOLTAMMETRY 9. However, the simple calculation was possible for the acetate buffer medium, because there is no significant change in parameter values with potential in this medium. The background reaction in this instance is not reduction of water, but of un-ionised acetic acid, and parameters had to be faked to simulate this change. This choice of parameters was ratified by matching computer-calculated scans with experimental scans: the fit was excellent, but the parameters listed in Table I have no real significance.The generation efficiency (or, rather, the loss of generation efficiency in parts per million) is plotted against current density in Fig. 6. The sharp decrease essentially marks the diffusion- limited current density. ” 2 4 6 8 10 12 14 16 18 Current density/mA Computed current efficiency loss in the generation of vanadium(1V) in the acetate buffer medium a t pH 4.0: “&”, cm; “[H+]”, 0.051 M ; “ k ~ ’ ’ , 1.61 x 10-l1 1 cm-2 s-l; 2.2 x 10-6 1 cm-2 s-’; k,,,, red, 2.2 x lo-’ 1 cm-’ s-’; K , 8-6 x Parameters in inverted commas are adjusted to give an exact computer fit with the experimental voltammograms Fig. 6. “OIH’’, 0.5; E’o, 0.525 v; [vv]B, lo-’ M ; [VIv]B, 0; n, 1; kmass ox, 1 cm-2 s-l; and a, 0.27. CONCLUSIONS The voltammetric iiivestigation of the vanadium(V) - vanadium( IV) reaction alone1 and in combination with manganese(VII), chromium(V1) and iron(II1) has permitted kinetic parameters to be evaluated, current efficiencies to be computed and a basis to be laid for the potentiostatic determination of vanadium(V) alone or in certain combinations with other elements. The adsorption of chromium(V1) and chromium( 111) on platinum at low hydrogen- ion concentrations has been demonstrated.August, 19731 KINETICS AND COULOMETRIC CURRENT EFFICIENCIES. PART VIII 571 We are deeply grateful to Imperial Chemical Industries for a research grant extending over 3 years. REFERENCES 1. 2. - - , Ibid., 1973, 98, 465. 3. 4. 5. 6. 7. Bishop, E., and Hitchcock, P. H., Analyst, 1973, 98, 553. Kolthoff, I. M., and El Din, A. M. S., J . Phys. Chem., 1956, 60, 1564. Davis, D. G., J . Electroanalyt. Ckem., 1960, 1, 73. Bishop, E., Analyst, 1972, 97, 761. Davis, D. G., Talanta, 1960, 3, 335. Bishop, E., Chemia Analit., 1972, 17, 511. NOTE-References 1, 2, 5 and 7 are to Parts VII, V, 111 and I, respectively, of this series. Received February 5th, 1973 Accepted March 27th, 1973

ISSN:0003-2654    

DOI:10.1039/AN9739800563

出版商:RSC    

年代:1973

数据来源: RSC

摘要:

572 Analyst, August, 1973, Vol. 98, +$. 572-579 Potentiostatic Coulometric Determination of Vanadium, Vanadium - Manganese and Vanadium = Iron Mixtures and the Influence of Chromium on the Process* B Y E. BISHOP AND P. H. HITCHCoCKt (Chemistry Departmeizt, University of Exetev, Stocker Road, Exeter, EX4 4QD) Earlier voltammetric work permitted the mass and charge transfer kinetic parameters of the vanadium system to be calculated for diverse media and platinum electrode pre-treatments, and command potentials to be selected for potentiostatic determination of vanadium alone and in certain combinations with other steel-forming elements. A simple coulometric cell and an adaptation of a commercial potentiostat are described. Current integration by strip-chart recorder is too inaccurate and so RC integration is discussed.Philbrick SP456 amplifiers refused capacitive feedback, but very satisfactory results were given by Solartron AA 1023 amplifiers. The design of a very high quality integrating capacitor from S.T.C. polystyrene elements is described; leakage and drift tests were very satisfactory. Pre- treatments of electrolytes and electrodes are discussed. Vanadium(V) is determined at -0.128 V in acetate buffer and at f0-247 V in 2.0 M sulphuric acid, in the latter with a relative standard deviation of 0.27 per cent. and a 95 per cent. confidence level result of (1.008 to 1.011) x 10-1 M compared with 1.012 x 10-1 iu for a standard solution. Chromium(V1) suppresses all reduction at pH 4.0, and is reduced simultaneously with vanadium in sul- phuric acid.Manganese(VI1) is reduced to manganese(II1) in the first step at +Om7 V at pH 3-5 and manganese(II1) and vanadiuin(V) are simultaneously reduced in the second step a t -0.12 V. The separation of iron(II1) is possible a t +O-9 V but impracticable; simultaneous reduction a t +0.25 V in 2.0 M sulphuric acid followed by re-oxidation oi the iron(I1) at + 1.0 V is recom- mended. A VOLTAMMETRIC study of the vanadium(V) - vanadium(1V) system in various media has been made in pure supporting electrolyte1 and also in mixtures with chromium(VI), man- ganese(VII), iron(II1) and combinations thereof . 2 Examples of current-efficiency calculations for the reduction of vanadium(V) have been reported, and a basis has been laid for the selection of conditions for the potentiostatic determination of vanadium(V) alone or in certain ad- mixtures2 Amperostatic coulometric determinations of vanadium have been reported in which such intermediates as ~opper(I),~ i r ~ n ( I I ) , ~ t i t a n i ~ r n ( I I 1 ) ~ ~ ~ and tin(I1)' were used, and Israel and Meites8 made a potentiostatic determination at mercury electrodes, but platinum electrodes do not seem to have been used for the potentiostatic determination of vanadium, either alone or in combination with the elements mentioned.Attempts so to do are described in this paper. EXPERIMENTAL Reagents and certain of the apparatus have been described el~ewhere.~ Potentials are quoted veYsus the standard hydrogen electrode (S.H.E.). COULOMETRIC CELL- The coulometric cell is shown in Fig.1. The anolyte and catholyte must be separated and diffusion of sample out of, or deleterious species into, the working compartment, in this instance the cathode, must be prevented. With the finest porosity sintered-glass separator, the auxiliary electrolyte was sucked into the working compartment by the stirrer. The junction was therefore immobilised with agar gel in saturated potassium sulphate solution. Tests showed, during a determination, no detectable migration of sample into the auxiliary compartment. As it is a protein, agar could act as a poison or deactivator for the working electrode,1° but no such action attributable to the agar occurred. * Presented at the Second SAC Conference, Nottingham, 1968. t Present address: Ever Ready Co. (G.B.) Ltd., Central Research Laboratory, St.Ann's Road, 0 SAC and the authors. London, N15 3TJ.BISHOP AND HITCHCOCK 573 Fig. 1. The coulometric cell and electrodes : C, cell consist- ing of a 400-ml beaker with top sawn off and ground flat; L, machined Perspex lid approp- riately drilled ; R, rubber sleeve holding the auxiliary elec- trode compartment; AE, auxiliary electrode, platinum gauze, JM 72050 ; RE, reference electrode (S.C.E.) with remote junction; WE, working electrode, platinum gauze, JM 72020; S, porosity 4 sintered-glass junc- tion plugged with agar gel; and M, PTFE-coated magnetic stirrer follower POTENTIOSTAT- A Wadsworth (Southern Analytical) electrogravimetric potentiostat was used for high- current work, modified so that the control potential could be set by means of a 39A pH meter; the command potential could be set much more precisely, and the high input impedance prevented polarisation of the reference electrode, which occurred with the unmodified instrument.CURRENT INTEGRATION- Initial trials were conducted by recording on a strip-chart recorder the decaying current as a function of the voltage drop across a standard resistor placed between the auxiliary electrode and the potentiostat. The current - time integral was obtained by cutting out the section of chart below the current trace, together with a square portion of paper of known area from above the trace, and weighing the paper on an analytical balance. The results were poor on account of the limitations of the recorder and variations in paper density, and the method was abandoned in favour of electronic RC integration by using chopper- stabilised operational amplifiers.574 BISHOP AND HITCHCOCK : POTENTIOSTATIC DETERMINATION OF [ArtabySt, VOl.98 There are three basic sources of error in RC integration: (1) the leakage resistance of the integrating capacitor, (2) the insulation of the amplifier summing junction, particularly to high voltages, and (3) the noise and drift of the amplifier itself. Morrison11 has examined some aspects of these errors, particularly (1) and (2). Regarding (S), little could be done; a t the time this work was carried out, only Philbrick SP456 and Solartron AA 1023 amplifiers were available. According to their manufacturers, both amplifiers were “capable of high- accuracy integration,” but neither manufacturer gave actual figures, particularly about the leakage resistance between the power supply and the summing junction.In the event, the Philbrick SP456 chopper amplifiers refused to operate with capacitance in the feedback loop, although they performed normally with resistive feedback. A second amplifier and different values of the input resistance and feedback capacitance were tried, and different earthing arrangements were tested in order to remove any unsuspected earth-loops, also without success. The standard arrangement of a single earth point, and that on the power supply, was adopted. The Solartron amplifiers performed very satisfactorily as integrators, and advantage was taken of their high-voltage output. As in the ramp generator de~ign,~ the current-measuring standard resistor was placed in the lead from the working electrode to the potentiostat, in parallel with a Solartron chopper amplifier in the current follower mode. The block diagram is shown in Fig.2. Calibration of the integrator was effected by substituting a constant-current source12 (either the Solartron AS 1411 or the operational amplifier source) for the cell and potentiostat, and integrating known currents for known times, which were measured by a crystal clock.12 For this purpose, the standard resistor, Rf, was a Cropico RS1 1.0-Q or a Cambridge 0.142 standard immersed in transformer oil, and the IR drop was measured with a Cropico P3 pot en t iometer . Fig, 2. The coulometric circuit: C, 1-pF poly- styrene integrating capacitor (see Fig.3) ; Q, Solartron AA 1023 chopper-stabilised amplifiers driven by Solartron AS 853.3 power supply (& 300 V, 100 mA) ; Rf, standard feedback resistor of current follower (0.1 Q); Ri input resistor to integrator (300 kQ); R1, load resistors (8.2 kQ) ; P, Wadsworth controlled potential apparatus ; V, Venner digital voltmeter; AE, auxiliary electrode ; RE, reference electrode; and WE, working electrode INTEGRATING CAPACITOR- The integrating capacitor is the most critical component in the circuit; it should be large, at least 1 pF, and have a minimum leakage resistance of 1013 Q, for an error of 0.1 per cent. over a 2-hour integration. A commercial source was long sought in vain, until the Development Department of the Capacitor Division of Standard Telephones and Cables Ltd.(S.T.C.) suggested a trial of high-quality, computer-grade, polystyrene capacitors, whichAugust, 19731 VANADIUM AND ITS MIXTURES WITH MANGANESE AND IRON 575 were then in the experimental stage. These were 0.2 pF -J= 1 per cent. S.T.C. 455/LWA/l12 FR capacitors of 500V d.c. working. They were tested by washing them with ethanol, suspending them in free air in a clean laboratory, charging them to 2 V and connecting them to the input of an E.I.L. 39A pH meter. For all samples, there was no detectable change over 2 hours; thereafter, for five samples the potential began slowly to decrease, at an in- creasing, although modest, rate as dust and moisture were deposited on them and produced surface leakage, and by the natural ionisation of the air caused by radioactive potassium contained in the materials of construction of the building.Another sample increased its charge slightly with time; this behaviour, although unusual, is not unknown, and came within the experience of the S.T.C. engineers, who could not offer a satisfactory explanation of its cause. The test in free air did not give a measure of leakage resistance but did show that it was satisfactorily low. The final design of the composite capacitor is shown in Fig. 3. Five capacitors, after thorough cleaning in ethanol, were mounted close-packed in parallel so as to give a capacitance of 1 pF. The remainder of the heavy machined material was thoroughly scrubbed with soap and water, washed with water and acetone and baked out before mounting.After assembly, the three-way tap was used for evacuation of the vessel and for leakage testing, and then to admit nitrogen dried over phosphorus(V) oxide. The evacuation and flushing operations were repeated many times, and the vessel was finally filled with dry nitrogen. Nitrogen was chosen as it has a higher excitation energy than argon. The whole assembly was protected from the atmosphere by a polythene bag. The terminals were connected to a Vibron pH meter and the capacitor was charged to 2.0 V. Readings were taken at intervals over a period of 300 hours, and showed that the combined leakage of capacitor, co-axial cable and the meter was 2.8 x 10l2 $2. If the leakage of meter and cable be taken as 1013 $2 (which is probably low), then the capacitor leakage resistance is about 3-9 x 1012sz* LIMITS IMPOSED ON INTEGRATION BY LEAKAGE- Consider the capacitor to be charged to 80 V in a determination.The leakage current at the end of the run is 80/(3-9 x 10l2) = 20 PA. An input current can therefore be summed to within 0.1 per cent. with a feedback capacitor leakage of 3.9 x lOl2Q. To the question of the loss that could be expected to occur after charging to 80 V and leaving for 1 hour, at such a large leakage resistance a sufficiently accurate answer is given by .. EC t I k a k = - where E is the loss in voltage, C is the capacitance and t is the time that remains before reading. Solving for E gives a value of 0.07 V, which represents an error of 0.09 per cent. in 1 hour, In tests of the completed integrator shown in Fig.2 , the drift was about 30 pV s-1, which is less than the manufacturer’s specification of 50 pV s-l for a unity gain integrator. Drift invariably increases with increase in the size of the input resistor, which explains the desire for a large capacitance. In the example of integrating for 1 hour to reach a final output of 80 V, the amplifier drift could cause an error as large as 0.108 V, or 0.13 per cent. GENERAL PROCEDURE FOR A COULOMETRIC DETERMINATION- The supporting electrolyte and any other necessary reagents were added to the working compartment. The large platinum-gauze electrode was pre-treated as required, and mounted in the cell. The auxiliary electrode and electrolyte were placed in the auxiliary compartment and the tip of the probe of the reference cell salt bridge was positioned near the bottom of the working electrode, as in Fig.1. This position was found to be the optimum both for reducing the IR drop in the command voltage and for minimising current noise occasioned by the stirring. After de-aeration of the electrolyte, a pre-electrolysis was performed until a constant current (the residual current) was attained. The integrator capacitor was dis- charged and the residual current integrated for a known time. The electrolysis was stopped, the integrator re-set and the sample added. Electrolysis was then continued until a constant current was again attained, the value of the current and the integrator voltage were noted and electrolysis was stopped. The integral of sample plus residual current was corrected for the residual current.The interpretation of the residual current correction is a severe limiting factor in potentiostatic co~lometry.~~576 [Analyst, Vol. 98 PRE-ELECTROLYSIS OF ELECTROLYTES- In instances in which the pre-electrolysis described above is known to deactivate the working electrode, or when an impurity can be removed by electrosorption, potentiostatic purification was conducted in a second sealed vessel with a platinum-black working electrode BISHOP AND HITCHCOCK : POTENTIOSTATIC DETERMINATION OF Scale - 0 1 2cm Scale I I I I I 0 1 2 3 4cm 1 r I r - I L " I '\ W - I I I \ \ \ H L I I Fig. 3. Sectional view of integrating capacitor: C, S.T.C. 455/LWA/112 FR polystyrene capacitors (0.2 pF f 1 per cent. 600 V d.c. working, five in parallel) ; N, 2 BA terminal nuts; S, platinum glass to metal seal; T, three-way greased tap; I, machined PTFE insulators; X, neoprene O-rings; and W, copper wire, 22 s.w.g.All parts are made of brass unless otherwise specifiedAugust, 19731 VANADIUM AND ITS MIXTURES WITH MANGANESE AND IRON 577 of geometric area 100 cm2. The all-glass cell carried ground-glass joints9 lubricated with distilled water, and bearing electrodes, a nitrogen bubbler and a glass siphon tube, the other end of which was led to the coulometric cell, After electrolysis to a satisfactorily small residual current, the “cleansed” electrolyte was forced into the voltammetric or coulometric cell by closing the nitrogen outlet so that electrolyte was forced out of the siphon tube.The siphon could be broken by an outlet tap at its highest point. The first few portions of electrolyte transferred were used to rinse out‘ the receiving cell, and were then discarded. POTENTIOSTATIC DETERMINATION OF VANADIUM (V)- Medium-From the voltammetric study,l the saturated potassium sulphate - acetate buffer at pH 4.0 and 2 . 0 ~ sulphuric acid are both suitable media for the determination. Both show well defined vanadium(V) limiting-current regions of sufficient potential range to permit the choice of a command potential that keeps the desired reaction well separated from background reactions so that a good current efficiency could be expected. Furthermore, in these media the ageing of the working electrode did not seriously affect the limiting-current region of the vanadium(V) wave.At intermediate hydrogen-ion concentrations, the deter- mination is impracticable, if not impossible, because severe deactivation of the electrode would so severely prolong the reduction that background corrections would be much too large to be reliable. Electrode pre-treatment1,2-Method (b)lv2 was selected and slightly modified on account of the large area of the working electrode; the previously cleaned electrode is immersed in 1.0 M sulphuric acid, anodised for 30 s at 100 mA cm-2 (i.e., 12-5 A) then cathodised for 30 s at 1 O O m A cm-2. The cycle is repeated once more, and the electrode thoroughly washed with water. That this modification made no difference to the electrode behaviour in the two media was confirmed by repeated voltammetric scanning, Selection of method (b) was made on the following grounds.Method ( a ) , reduction with iron(I1) in sulphuric acid, had not proved particularly successful, and was suspect in respect of transfer of trace amounts of iron(I1) or iron(III), trapped in the gauze meshes, into the test s01ution.l~ Method (e), oxidation, was deemed unsuitable because the charge-transfer process was slowed down by this treatment, and an error would be introduced by the reduction of the oxide film, although a correction could be applied for the latter. Methods ( b ) , (c) and (d) are very similar, but method (d) , simple cathodisation, was considered less suitable because such electrodes appear to be prone to cumulative adsorption of some species in the vanadium sample on repeated use.Such an impurity is known to be removed by anodisation, and so method (b) was chosen, in preference to method (c), stripping and cathodisation, as being simpler and faster. Method (c) remains appropriate for the initial cleaning of dirty electrodes. Determinations-The results of determinations in the two chosen media with electrodes pre-treated by method (b) are collected in Table I, and show that results obtained in 2-0 M sulphuric acid are substantially better than those for the buffer at pH 4.0. This effect arises from two causes. First, different integration methods were used, and the strip-chart record- ing - weighing method for group D is not as accurate as the electronic method. The second cause is more fundamental and important. In acetate buffer, the background current, which was substantial, changed during the determination, but the exact manner of the change is not known and therefore cannot be accurately corrected f0r.1~ There was little alternative but to use a mean value of the pre-titration and post-titration background currents, and the error is magnified by the low charge-transfer rate and the long duration of the determination of 2 to 2.5 hours.In both media, graphs of log current veYsz4s time were linear. The increased determination time in the buffer of pH 4 is aggravated by the lower mass-transfer rate constant2 in this medium. POTENTIOSTATIC DETERMINATION OF VANADIUM(V) IN THE PRESENCE OF OTHER METALS- Chromium( VI)-In acetate buffer, chromium(V1) is specifically adsorbed on the electrode surface and quantitatively blocks the vanadium reduction.2 In 2.0 M sulphuric acid, van- adium(V) and chromium(V1) are reduced at virtually identical rates, giving a single wave.2 In this medium, the total of vanadium film chromium can be determined at a command potential of +045 V, but the current efficiency for the reduction of chromium(V1) is unlikely to be high.RESULTS AND DISCUSSION578 BISHOP AND HITCHCOCK : POTENTIOSTATIC DETERMINATION O F [Analyst, VOl. 98 TABLE I RESULTS OF THE POTENTIOSTATIC DETERMINATION OF VANADIUM (V) ALONE Group Conditions A Saturated potassium sulphate - acetate buffer a t pH 4.0, electronic integration, command potential -0.128 V B 2.0 M sulphuric acid, electronic integration, command potential + 0.247 V C 2.0 M sulphuric acid, strip-chart record integration, command potential + 0.247 V D Saturated potassium sulphate - acetate buffer a t pH 4.0, strip-chart record integra- tion, command potential -0.128 V Sample takenlmol 2 x 10-3 2 x 10-3 2 x 10-3 2 x 10-3 2 x 10-3 2 x 10-3 2 x 10-3 2 x 10-3 2 x 10-3 2 x 10-3 2 x 10-3 2 x 10-3 1 x 10-3 5 x 10-4 1 x 10-4 2 x 10-3 2 x 10-3 2 x 10-3 2 x 10-3 3 x 10-3 5 x 10-3 5 x 10-4 Background current Initial/mA Final/mA 0.2 0.1 0.15 0.1 0.09 0.1 7 0.15 0.15 0.35 0.15 0.05 0.04 0.05 0-15 0.10 0.10 0.15 0.07 0.36 0.41 0.15 0.15 0.24 0.05 0.10 0-05 0.05 0.04 0.10 0.05 0.15 0-08 0.52 0-50 0.47 0.5 1 0.18 0.37 0.34 0.21 0.27 0.44 0.13 0-37 Time1 minutes 130 140 145 84 71 102 99 81 83 160 140 80 76 81 83 75 88 120 110 130 160 117 Relative error, per cent. + 5-1 + 1-54 + 2.04 +O.lS - 0.51 -0.21 + 0.07 - 0.45 - 0.35 -0.87 - 0-10 -0.10 - 0.20 - 0.28 -0.11 - 0.93 - 0.53 - 1.5 -3.1 +3-7 +2*1 - 1.0 The first nine results in section B give a relative standard deviation of 0.27 per cent.For the 95 per cent. confidence level, this value indicated that the stock vanadium(V) solution was (1.008 to 1.011) x lo-' M. Standardisation by means of sulphur dioxide and permanganate against sodium oxalateB gave a value of 1.012 x 10-1 M. Mmganese(VII)-Well separated waves are obtained in the acetate buffer at pH 4~0,~ and on this basis the sequential determination of manganese(VI1) at +0.700V and van- adium(V) at -0.125 V was attempted. As earlier adumbrated,2 the quantities of electricity consumed by the two steps showed that they corresponded first to the reduction of man- ganese(VI1) to manganese(III), at +O-7 V, and second to the simultaneous reduction of manganese(II1) to manganese(I1) and of vanadium(V) to vanadium(1V) at -0.125 V.Repli- cate experiments showed that the results were inadequately consistent, and also that some manganese(1V) oxide was formed by disproportionation of manganese(II1). To depress this latter effect, the pH was decreased to 3-5 by the addition of acetic acid to the medium. Determinations made in this new medium showed that the first stage in the reduction was improved in reproducibility and accuracy, with a relative error of about h0.3 per cent., as shown in Table 11. The second stage in the reduction remained in error by about +%O per TABLE I1 RESULTS OF THE SEQUENTIAL POTENTIOSTATIC DETERMINATION OF MANGANESE(VII) AND VANADIUM(V) IN SATURATED POTASSIUM SULPHATE - ACETATE BUFFER AT pH 3.5 Electronic integration was used Sample takenlmol MnvII vv' 5 x 10-4 2 x 10-3 5 x 10-4 2 x 10-3 5 x 10-4 2 x 10-3 5 x 10-4 4 x 10-3 2 x 10-3 1 x 10-3 2 x 10-3 1 x 10-3 3 x 10-3 i x 10-3 Command potential/V 7 MnvII VV 0.697 - 0.120 0.697 - 0.120 0.697 -0*110 0.697 -0*110 0.697 -0~110 0.697 -0.120 0.697 -0-120 Relative error, per cent.7 MnvI1 VV +0*31 + 1.1 + 0.10 + 1.7 +0.11 - 1.0 -0.21 + 0.8 -0.19 -2.1 + 0.09 - 1.0 - 0.27 + 1.6August, 19731 VANADIUM AND ITS MIXTURES WITH MANGANESE AND IRON 579 cent. after correction for the background current. However, the simple determination of vanadium(V) in acetate buffer at pH 4.0 was also in error by this amount, as shown in Table I, so the error in the sequential determination was not unexpected.The two-step reduction of manganese(VI1) was not predicted from the voltammetric curves2 because the mass-transfer rate constant was not independently known for the conditions used. The voltammograms2 showed only a single step for manganese(VI1) reduction, and although the limiting current could be measured to within 1 per cent., knowledge of one other parameter would be needed in order to calculate the n-value of the wave. A determina- tion on this mixture was not attempted in 2.0 M sulphuric acid, because the voltammograms2 showed that the wave separation was inadequate for good separation efficiency, although the sum could be determined with good accuracy and efficiency.Imn(lll)-The voltammogram2 suggests that, by using electrodes activated by methods ( b ) or (C),~ it should be possible to reduce vanadium(V) at a command potential of +O-9 V without reducing an appreciable amount of iron(II1). Even if some iron(I1) were formed, it would act as an intermediate and reduce vanadium(V) chemically. Experimentally, it was found that, in 2 . 0 ~ sulphuric acid, the current decayed rapidly to about one third of its initial value during the first 10 minutes and thereafter the decay became much slower. A graph of log current veysus time for the first part of the reduction was not linear, but convex to the time axis. This result indicated that the electrode surface was being deactivated t o the type of surface used to record scan 2 of Fig.4 in reference 2. Although it remained possible to reduce the vanadium(V) without appreciable reduction of iron(II1) by continuing the potentiostatic electrolysis until the current decreased to an acceptable residual value, the time required would be so excessively long that the background correction would be considerable and the determination would be of poor accuracy. Alternatively, it is possible to reduce both vanadium(V) and iron(II1) simultaneously and quantitatively in 2.0 M sulphuric acid at +0.25 V, and then to re-oxidise the iron(I1) at + 1.0 V without any risk of re-oxidising the vanadium(1V) or the solvent, and so obtain the vanadium concentration accurately by difference. Unfortunately, the latter part of this procedure could not be attempted, as the only high-current potentiostat available at the time would operate only in the cathodic mode. CONCLUSIONS The potentiostatic determination of vanadium(V) is possible in the two media selected, and the results obtained in 2-0 M sulphuric acid by using electronic integration are both precise and accurate.The sequential potentiostatic determination of manganese(VI1) and van- adium(V) in acetate buffer at pH 3.5 is possible, although the mechanism is unexpected in that the first step is reduction to manganese(III), which is then reduced to manganese(I1) simultaneously with the reduction of vanadium in the second step. Chromium(V1) can be reduced simultaneously with vanadium(V) , but not sequentially. Reduction of vanadium(V) in the presence of iron(II1) is possible but not practicable, and it is better to reduce both and then to re-oxidise the iron(I1); manganese can be added to this combination. We are deeply grateful to Imperial Chemical Industries Limited for a research grant extending over 3 years. We thank the Capacitor Division of S.T.C. Ltd. for the gift of polystyrene capacitors from which the integrating capacitor was constructed. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. REFERENCES Bishop, E., and Hitchcock, P. H., Analyst, 1973, 98, 553. Meier, D. J., Myers, R. J., and Swift, E. H., .J. Amer. Chem. S O ~ . , 1949, 71, 2340. Furman, N. H., Reilley, C. N., and Cooke, W. D., Analyt. Chem., 1951, 23, 1665. Kennedy, J. H., and Lingane, J. J., Analytica Chinz. Acta, 1958, 18, 240. Lingane, J. J., Ibid., 1956, 15, 465. Anson, F, C., and King, D. M., Analyt. Chem., 1962, 34, 362. Israel, Y., and Meites, L., J . Electvoanalyt. Chem., 1964, 8, 99. Bishop, E., and Hitchcock, P. H., Analyst, 1973, 98, 465. -- , Ibid., 1973, 98, 475. Morrison, C. F., “Generalised Instrumentation for Research and Teaching,” Washington State Bishop, E., and Riley, M., Analyst, 1973, 98, 305. Meites, L., and Moros, S. A., Analyt. Chem., 1959, 31, 23. Bishop, E., and Riley, M., Analyst, 1973, 98, 416. I , Ibid., 1973, 98, 563. -- University, Pullman, 1964. Received February 19th, 1973 Accepted March 20th, 1973

ISSN:0003-2654    

DOI:10.1039/AN9739800572

出版商:RSC    

年代:1973

数据来源: RSC

摘要:

580 Aaalyst, August, 1973, Vol. 98, pp. 580-584 A Multi-channel Dispenser - Titrator - pH-stat* BY DOUGLAS G. MITCHELL AND KENNETH M. ALDOUS (Division of Laboratories and Research, New York State Department of Health, Albany, New York 12201, U.S.A.) A multi-channel dispenser - titrator - pH-stat with switch-selection of reagents and reagent volume control and with a directly digital read-out is described. A valvc driven by a stepper motor is used to select reagents and as a coarse volume control. A drop generator dispensing - deflection mechan- ism is used as a fine volume control. The drop generator is also used to reject automatically to waste any reagent contaminated by previously dispensed reagent and, in the titration mode, to add various amounts of reagent to the reaction mixture.The instrument gives outstanding precision (relative standard deviation less than 0.2 per cent. over the range 0.5 to 7.0 ml) and linearity (correlation coefficient Y = 0.999 over the ranges 0.01 to 1.0 and 0.25 to 7.0 ml). THE dispensing and, to a lesser extent, titration of liquids are basic operations in all wet- chemical analytical procedures. The most common dispensing operations can be conveniently grouped into fixed-volume and variable-volume sample and reagent dispensing. Efficient instrumentation is available for both fixed-volume operations, and some dispenser - diluters can handle variable-volume sample dispensing, but no fully automated instrumentation is available for variable-volume reagent dispensing (Table I). Also, none of the available automatic dispensers are also capable, without modification, of being used in automatic titrations and pH-statting procedures.TABLE I INSTRUMENTATION FOR AUTOMATED DISPENSING Dispensing operation Available instrumentation Manual : Pipettes, burettes and syringes Automated : Syringe dispensers and diluters Fixed-volume sample dispensing Variable-volume sample dispensing Manual : Pipettes, burettes and syringes { Automated : Some dispenser - diluters Manual : Pre-set pipettes fitted on to reagent containers, burettes, pipettes and syringes Automated : Probably unnecessary Fixed-volume reagent dispensing Variable-volume reagent dispensing , " , a r b u , " a ; ~ r ~ ~ ~ ~ ~ d ~ ~ ~ ~ ~ e d instrumentation available In this paper, we describe a dispenser - titrator - pH-stat based on the use of a droplet generator, the liquid being dispensed into a stream of uniformly sized and uniformly spaced droplets, as a first step in the dispensing or titrating operation.Droplet generator devices have been previously used to dispense liquids. For example, Schneider and Hendricksl and Schneider, Lindblad and Hendricks2 have used a droplet generator to study the collision and coalescence of liquid aerosols; Sweet3 has described a high-frequency ink writer; Hieftje and Malmstadt4 used a droplet generator for sample introduction in flame spectrophotometry ; and Hieftj e and Manadarano5 developed a single-channel titrator. A single-channel micro- dispenser (Princeton Fluidics Corporation, Princeton, N. J., Model 390) has also been used.These instruments are not directly applicable to dispensing and titrating operations in a routine analytical laboratory because of the considerable practical difficulties in changing reagents and because of the lack of a coarse reagent on - off control to prevent wastage. The instrument described herein can dispense or titrate six reagents with dial selection of both reagent and reagent volume. It is also fitted with several devices needed for a routine analyti- cal instrument: a reagent thermostat in order to eliminate variations in volume dispensed because of viscosity changes, a coarse reagent on - off control and a flexible mode of approaching the end-point, this mode being selected by the operator. * Presented in part a t the Pittsburgh Conference, Cleveland, Ohio, March, 1972. @ SAC and the authors.MITCHELL AND ALDOUS 581 PRINCIPLE OF OPERATION- The basic dispensing system is shown schematically in Fig.1. Reagents are placed in containers, pressurised at about 11 p s i . , and connected to the entrance ports of a six-way valve driven by a stepper motor. The appropriate reagent can flow through this valve, through a thermostatically controlled coil, and then through a vibrating needle, from which it emerges as a stream of uniformly sized drops. These drops are either electrostatically charged (by applying +a00 V to the cylindrical charging electrode) or left uncharged (elec- trode at 0 V) and are then passed between two highly charged deflecting electrodes ( 5 3 kV). Charged drops are electrostatically deflected and allowed to drain to waste, while uncharged drops are allowed to proceed undeflected into the desired container.The flow-rate of the liquid is constant at constant pressure within the reagent vessel and temperature of the liquid; hence the volume dispensed can be controlled by selecting the time during which the liquid is allowed to proceed into the container. Mu Iti-port valve fi ~ To other containers reagent Needle +400V or OV To To digital pH meter Fig. 1. Basic dispensing system A dispensing operation is carried out by setting a switch to select the liquid to be dis- pensed, setting a digital switch to select the reagent volume and pressing a start button. The following events then occur in sequence: 1. The stepper motor moves the valve from “off” to the pre-selected position, thus allowing the reagent to flow.2. All liquid flowing during the first few seconds is dispensed to waste (as it is presumed t o be contaminated by previously dispensed reagent). 3. Reagent is allowed to proceed undeflected into the desired container for the time required to give the selected volume. 4. The liquid stream is deflected to waste for a few milliseconds so as to avoid switching irregularities. 5. The valve is returned to the “off” position. Thus the valve acts as a reagent selector and as a coarse on-off control, and the drop generator mechanism acts as a fine volume control and an automatic washing device to prevent cross-cont amination.582 MITCHELL AND ALDOUS : A MULTI-CHANNEL [Analyst, Vol. 98 The same basic mechanism is used for titration and pH-statting procedures, except that a digital pH meter with further control circuitry is used to control the dispensing operation.The pH meter output is fed to comparator circuitry, which senses the difference between the pre-selected pH and the pH of the reaction mixture. Titrant is dispensed at the maximum rate of approximately 8 ml min-1 until a pre-selected pH is reached. At this stage, dispensing is stopped for a selected period of between 0 and 20 s so as to allow sufficient time for the chemical reaction to proceed. (This step is necessary, for example, when titrating natural waters with a strongly acidic titrant.) Thereafter, dispensing proceeds at a selected rate of 100, 80, 60, 40, 20 or 10 per cent. of the maximum rate until the pre-selected pH is reached and the valve is turned off, Subsequent changes of pH, as in pH-statting operations, will cause the valve to open, thus allowing further reagent to be dispensed as required in order to maintain the selected pH value.The circuitry necessary to carry out these operations is shown schematically in Fig. 2. INSTRUMENT COMPONENTS- The high-voltage power supplies (400 V, less than 5 per cent. ripple; 3 kV, less than 10 per cent. ripple) and digital timing circuitry were built up from standard electronics components. Suitable components (valve, thermostatically controlled coil, reagent flasks and dispensing heads) for handling liquids were not available commercially and were constructed in our workshop. The thermostatically controlled coil comprises a 5-foot coil of 0-017 inch i.d.polypropylene tubing encased in a metal block and maintained a t 30 & 0.1 "C. A Bellingham, Model 123, air pressure regulator was used to regulate the air pressure in reagent flasks, and a Leedex 170-733-001 stepper motor was used to drive the reagent selector valve. Mode: Dispense, repeat, titrate Digital switch pH meter --------- --- Comparator Rate of approaching end-point Valve position motor switch I I - Stepper - electrode I II Mu Iti-port valve 1 driving oscillator plates Fig. 2. Schematic diagram of digital control and count- ing circuitry for multi-channel dispenser - titrator systemAugust, 19731 DISPENSER - TITRATOR - PH-STAT INSTRUMENT OPERATION- 583 The basic operating procedures are as follows.Dispense - Yepeat dis$ePzse-(i) Select reagent; (ii) dial volume; and (iii) press “Start.” Titvate - $M-stat-(i) Select titrant; (ii) fit pH meter and electrode; (iii) select pH (mV) at end-point; (iv) select pH (mV) at reaction pause; (v) select reaction pause time; ( v i ) select rate of approaching end-point; (vii) press “Start”; and (viii) read titrant volume on display. RESULTS DISPENSING MODE- Precision-The precision was evaluated by dispensing ten successive volumes of distilled water and weighing the dispensed liquid. Relative standard deviations of less than 0.2 per cent. were obtained over the range 0-5 to 7.0 ml, which is exceptionally good precision, particularly as there is no possibility of “drop at the dispenser tip” error as with conventional syringe dispensers.Most of the variation in dispensed volume is associated with the mechanical switching operation, because if the valve is locked in the “on” position, values for relative standard deviation of better than 0.05 per cent. can be obtained over this dispensing range. TABLE I1 PRECISION RESULTS FOR REPETITIVE DISPENSING The results are shown in Table 11. Volume dispensed/ml Relative standard deviation, per cent. 0.5 0.19 1.0 0-17 3.0 0.19 7.0 0.10 Linearity-Volumes dispensed over the ranges 0.01 to 1.0 and 0.25 to 7.0 ml were Correlation coefficients of 0.999 measured and plotted against instrument setting (Fig. 3). were obtained for both ranges. Volume selected/ml Fig. 3. Calibration graphs TITRATION MODE- Analytical precision in the titration mode was evaluated by connecting the instrument to a digital pH meter (Model NX, Sargent Welch Scientific Co., Skokie, Illinois 60076), by using a glass and platinum electrode as required (Model 117233; Leeds and Northrup, North Waks, Pa. 19545).Precision results were obtained for strong acid - strong base, strong acid - weak base and redox titrations. These results are shown in Table 111.584 MITCHELL AND ALDOUS TABLE I11 TITRATION RESULTS : MULTI-CHANNEL DISPENSER - TITRATOR - pH-STAT Sample End- volume/ point Sulphuric acid (0.02 N) Sodium hydroxide 10.0 pH 8.6 Sulphuric acid (0.2 N) Potable water 10.0 pH 4.5 Titrant Sample ml selected solution (approx. 0.02 N) Ammonium cerium(1V) Ammonium iron(I1) 10.0 8.6 mV sulphate solution sulphate solution (0.025 M) (0.025 M) Relative standard Mean deviation titrant of titrant volume/ volume, Titration ml per cent.time/s 9.23 0.14 150 0.47 3.3 50 10.09 0.74 150 DISCUSSION The instrument described provides an exceedingly efficient means for carrying out the numerous dispensing and titrating procedures required in most laboratories in which wet- chemical methods are used. It has the major advantage of dial selection of both reagent and reagent volume, with excellent precision, linearity and accuracy, and a directly digital read-out. The instrument is currently being used in our water analysis laboratory for preparing standard solutions €or atomic-absorption spectrophotometry and colorimetry, for neutralising reaction mixtures after acid digestion, for acid - base titrations and for various other dis- pensing operations. The instrument maintains calibration for the dilute reagents used in our laboratory but not for concentrated reagents such as 1 M sodium hydroxide solution and organic solvents, which have viscosities that are significantly different from that of distilled water; these reagents require re-calibration. Calibration can be readily checked by dispensing liquid into a 1-0-ml standard flask. As the instrument is completely linear, this check suffices for the whole calibration range. The directly digital read-out is an additional advantage. We shall shortly interface the dispenser - titrator and digital pH meter directly with a laboratory mini-computer. The authors thank Mr. Gordon Patrie for his excellent technical assistance. REFERENCES 1. 2. 3. 4. 5. Schneider, J. M., and Hendricks, C. D., Rev. Scient. Instrum., 1964, 35, 1349. Schneider, J. M., Lindblad, N. R., and Hendricks, C. D., J . Colloid Sci., 1966, 20, 610. Sweet, R. G., Rev. Scient. Instrum., 1965, 36, 131. Hieftje, G. M., and Malmstadt, H. V., Analyt. Chem., 1968, 40, 1860. Hieftje, G. M., and Rlanadarano, B. M., Ibid., 1972, 44, 1616. Received August 23rd, 1972 Amended March Znd, 1973 Accepted March 14th, 1973

ISSN:0003-2654    

DOI:10.1039/AN9739800580

出版商:RSC    

年代:1973

数据来源: RSC

摘要:

Artalyst, August, 1973, Vol. 98, 99. 585-592 585 A Modified Field Test for the Determination of Carbon Disulphide Vapour in Air BY E. C. HUNT, W. A. McNALLY AND A. F. SMITH (Department of Trade and Industry, Laboratory of the Government Chemist, Cornwall House, Stamford Street, London, SE1 9NQ) An improved and more sensitive method is described for the determination of carbon disulphide vapour in air a t concentrations up to 40 p.p.m. V/V. Carbon disulphide vapour is absorbed from a 500-ml sample of air into an ethanolic solution containing copper(I1) acetate, diethylamine and triethanol- amine. The yellow colour produced is compared visually with standard colours or measured spectrophotometrically. For field use, the apparatus is portable and simple to operate, and requires a working time of about 5 minutes per determination.THE field test currently in use by H.M. Factory Inspectorate for the determination of carbon disulphide vapour in air1 is basically the same as that which was first published in 1939 and is derived from a method2 originally published in 1932. The method is required for assessing the atmosphere within a factory and to give a rapid, approximate determination of concen- trations of carbon disulphide in the region of the threshold limit value, currently 20 p.p.m. V/V,3 in order to judge whether or not a hazard to health exists. (Threshold limit values refer to time-weighted concentrations in air for an 8-hour working day and a 40-hour working week, and represent conditions under which it is believed that nearly all workers may be repeatedly exposed without adverse effect on their health.Detailed conditions and exceptions are given in reference 3.) A re-examination of the method was undertaken for the following three reasons. (i) It was occasionally found that when the component solutions were mixed in the bubbler the absorption solution became turbid and its colour blue instead of remaining a clear, pale green. (ii) The volume of air to be sampled (2.5 litres) was of an inconvenient size; it was excessively large for the use of a rubber-bulb aspirator, which would require twenty aspira- tions, and too small to warrant the use of an electric pump and flow meter. The rubber-bulb aspirator was preferred because of its portability, and so an increase in sensitivity was sought in order to permit a reduction in the volume of air sampled.(iiz) The existing method was devised before threshold limit values for toxic substances in air were first compiled and it does not provide for a standard at the threshold limit value for carbon disulphide. A survey of published analytical methods disclosed no method that appeared suitable for adaptation as a replacement for the existing method, and modification of the latter seemed to afford the most profitable line of investigation. EXPERIMENTAL The current method requires the contaminated atmosphere to be passed through a bubbler containing 10 ml of ethanol, 2 ml of a 2 per cent. V/V solution of diethylamine in benzene and 2 ml of a 0.1 per cent. m/V solution of copper(I1) acetate in ethanol; the colour of the copper diethyldithiocarbamate formed is allowed to develop for 20 minutes.For use as a field method, the amount of carbon disulphide collected in the absorption solution is determined by visually comparing the colour formed in this solution with colour standards. The first steps taken to improve the sensitivity were to reduce the concentration of copper(I1) acetate in the absorption solution from 0.014 to 0.002 per cent. m/V, to eliminate the benzene by preparing the diethylamine solution in ethanol and to reduce the total volume of the absorption solution by adding only 6 ml of ethanol instead of 10 ml. These changes, and the evaluation of the colour by using a longer light path, produced a satisfactory increase n sensitivity, with the added advantage that the reagent blank was colourless instead of @ SAC; Crown Copyright Reserved.586 HUNT, MCNALLY AND SMITH: A MODIFIED FIELD TEST FOR [Amlyst, Vol.98 pale green. During the development of the method, colours were measured spectrophoto- metrically4 at 430 nm after allowing the colour to form for 20 minutes. The optical density was found to reach a maximum at this wavelength after about 2 minutes (Fig. 1) but the colour changed visibly from lemon yellow to amber during the subsequent 18 minutes. The absorption spectrum of the solution, recorded at intervals (Fig. l), showed initially a single peak with ix maximum at 440 nm. Within 5 minutes, the peak had shifted to 457 nrn and a shoulder had begun to appear at about 385 nm, which developed into a peak and increased in height for at least 30 minutes.The peak a t 457 nm reached a maximum during this time. 0.5 1 0.4 3 50 400 450 500 Wavelength/nm Fig. 1. Variation of the absorption spectrum with reaction time. Carbon disulphide (60 p g ) added to ethanol containing 0.002 per cent. m/V of copper(I1) acetate and 0.4 per cent. V / V of diethylamine. Re- action time: A, 1 ; B, 2 ; C, 5; D, 15; and E, 30 minutes EFFECT OF WATER- It was found, during tests with the current field method for determining carbon di-- sulphide, that the addition of a small amount of water to the absorption solution produced turbidity and a change in colour from pale green to pale blue. Application of the test under factory conditions, with possibly perfunctory washing and drying of the apparatus, could lead to retention of moisture, especially as sintered-glass bubblers are used.Similar effects were found with the modified absorption solution described above when water was added. The modified solution was prepared in absolute ethanol (maximum water content 0-5 per cent. V / V ) and contained no benzene. Small amounts of water were added to the ethanol used in preparing the reagent solutions in order to study the effect on colour in more detail. Absorption spectra recorded after the addition of 60 pg of carbon disulphide to the absorption solution, and allowing 30 minutes for reaction time, showed that as the water content increased, the peak at 385 nm was reduced and the main peak at 457 nm shifted to 435 nm. With additions of water to concentrations greater than 5 per cent.V/V, only the peak at 435 nm remained (Table I). At these levels of water concentration, the copper(I1) acetate solution deteriorated rapidly, turning dark brown and becoming turbid. Replacement of ethanol with butan-2-01 or 2-methoxyethanol has been recommended5 in order to eliminate this effect but was found to be unsatisfactory because of the formation of coloured reagent blanks that seriously impaired visual colour comparison. Other workers6y7 had added tri- ethanolamine to the absorption solution but, when it was added to the solution used in the current field method, the blank solution became blue in colour. However, a stable colour with an absorption maximum at 430 nm was obtained in the presence of carbon disulphide, which was not affected by additions of water in the range 1 to 10 per cent.V/V. With the modified absorption solution, which contained less copper(I1) acetate, the reagent blank wasAugust, 19731 THE DETERMINATION OF CARBON DISULPHIDE VAPOUR IN AIR TABLE I EFFECT OF VARIATION OF WATER CONTENT ON THE COLOUR 587 Carbon disulphide (60 pg) in 10 ml of ethanol containing 0.002 per cent. m/V of copper(I1) acetate and 0.4 per cent. V/V of diethylamine after colour development for 30 minutes Water content, Optical density per cent. V/ V Absorption maximumlnm (10-mm cells) 0.5 385 0.300 457 0-460 1.0 385 0.250 457 0.455 1.5 385 0.200 457 0.450 2.0 448 0.450 2.5 442 0.470 3.0 440 0.480 3.5 440 0-495 4.0 440 0.490 4.5 440 0.505 5.0 435 0.500 10.0 435 0.500 20.0 435 0.510 Optical densities a t 385 nm remained constant at 0-200 for water contents of 1-5 per cent.or more. reduced to an acceptable level without unduly affecting the sensitivity of the test. A con- centration of 1.0 per cent. V/V of triethanolamine in the absorption solution was chosen because, at this level, small errors in volume measurement, caused by the viscous character of the liquid, would not seriously affect the final colour or the sensitivity of the test (Table 11). With the incorporation of triethanolamine, all the reagents could be combined in a single solution that was stable for at least 1 month, and the more widely available industrial methylated spirit (water content about 5 per cent. V/V) could be used instead of absolute ethanol. TABLE I1 EFFECT OF ADDITION OF TRIETHANOLAMINE Carbon disulphide (60 pg) in m/V of copper(I1) acetate and 10 ml of 96 per cent.V/V ethanol containing 0.002 per cent. 0.4 per cent. V/V of diethylamine after colour development for 5 minutes Triethanolamine content, per cent. V/V 0 0.2 0.4 0.8 1.2 1.6 2.0 4.0 Optical density a t 430 nm (10-mm cells) 0.492 0.461 0.455 0.455 0.455 0-45.5 0.450 0.440 PREPARATION OF A STANDARD ATMOSPHERE- An atmosphere containing a known concentration of carbon disulphide was required to assess the efficiency of sampling and of the chemical reactions that took place during the application of the field method. The atmosphere was prepared dynamically by an injection - atomisation technique in which carbon disulphide was injected at a known rate into a metered air stream.Part of this atmosphere was diluted with a second metered air stream to give the required concentrations. (The standard atmosphere generator was designed to ensure homogeneity and was checked at several different concentrations by a chemical method other than the described field test and found to give reproducible results.) The generated atmos- phere was calibrated by collecting samples at the rate of 125 ml min-l for 8 minutes in three all- glass bubblers in series, each containing 10 ml of ethanolic potassium hydroxide solution (0.2 per cent. m/V). The xanthate formed in each bubbler was determined spectrophotometrically588 [Analyst, vol. 98 as its copper salt.* With atmospheres containing up to 40 p.p.m. V/V of carbon disulphide, no colour developed in the last bubbler.COLLECTION OF CARBON DISULPHIDE- The efficiency of absorption of carbon disulphide from air with the simple bubbler and absorption solution described in the Procedure was found to be 100, 94 and 89 per cent. at flow-rates of 50, 125 and 200 ml min-l, respectively, for atmospheric concentrations in the range 0 to 40 p.p.m. V/V. The effect of temperature on absorption efficiency was found to be small and was linear between values of 96 per cent. at 5 "C and 91 per cent. at 35 "C, at the prescribed flow-rate of 125 ml min-l. RELIABILITY OF REAGENTS- A comparison was made of reagents used in the test that were obtained from various manufacturers and of different batches from the same manufacturer. Negligible differences were found. HUNT, McNALLY AND SMITH: A MODIFIED FIELD TEST FOR INTERFERENCES- Hydrogen sulphide is the substance most likely to be found as an industrial co-con- taminant with carbon disulphide and is known to give a positive reaction with the proposed test.A hydrogen sulphide atmosphere, prepared by an injection technique, was stan- dardised by an iodimetric method. Samples of this atmosphere at concentrations of 10, 20 and 40 p.p.m. V/V were taken by the proposed field method and were found to give colours equivalent to 3, 5 and 10 p.p.m. of carbon disulphide, respectively. The current method for determining carbon disulphide in air1 recommends the use of filter-paper im- pregnated with lead acetate to remove hydrogen sulphide and this method, together with the use of similarly impregnated cotton-wool, was investigated for use with the revised test.The introduction of a filter into the sampling system considerably reduced the flow-rate and also the effective volume sampled when a rubber-bulb aspirator was used, hence it was essential that the flow-rate of the aspirator be adjusted before use, with both filter and bubbler in circuit. Once adjusted, no significant change in flow-rate was noted when the filter-paper was changed. Cotton-wool, however, gave variable flow-rates, depending on its packing density; consequently, its use as a hydrogen sulphide trap was abandoned. Samples of the standard hydrogen sulphide atmosphere at concentrations of 10, 20 and 40 p.p.m. V/V were taken by the proposed method with a pre-filter of impregnated paper.Colours equivalent to 1, 2 and 4 p.p.m. of carbon disulphide, respectively, were obtained by using Whatman No.1 filter-paper and 0, less than 0.5 and 1 p.p.m. by using Whatman 3MM paper; the latter was chosen as suitable for use in the test. By mixing the outputs from the two generators in suitable proportions, a mixed atmos- phere was obtained, which was sampled both with and without a pre-filter prepared from Whatman 3MM paper. The average results obtained are shown in Table 111. TABLE I11 INTERFERENCE EFFECT OF HYDROGEN SULPHIDE ON THE FIELD METHOD Tests with standard atmospheres with and without lead acetate filters Found by field test, p.p.m. V/V of carbon disulphide Atmospheric concentration, p.p.m. V / V Carbon disulphide Hydrogen sulphide With filter Without filter 22.5 0 22-5 22.5 0 8.7 0 2.3 22.5 8.7 22.5 24-8 A r A > r 1 PREPARATION OF COLOUR STANDARDS- Colour standards were prepared by adding aliquot portions of an ethanolic solution of carbon disulphide to the absorption solution, allowance being made for volume changes and the fact that only 94 per cent.of the carbon disulphide present in the atmosphere is collected under the test conditions. Once prepared, the standards were found to be stable for at least 4 hours, provided that they were kept in well stoppered tubes. Details of the preparationAugust, 19731 589 of these standards are given later. Results obtained when carbon disulphide atmospheres of known concentration were sampled by the proposed method are given in Table IV. The levels were assessed visually by using the colour standards and, more accurately, with a spectrophotometer.The optical densities of the solutions were measured at 430 nm and the carbon disulphide concentration was determined by reference to a prepared calibration graph. TABLE IV EVALUATION OF FIELD METHOD WITH STANDARD ATMOSPHERES THE DETERMINATION OF CARBON DISULPHIDE VAPOUR IN AIR Concentration of carbon disulphide, p.p.m. V/ V i- A > Found by field method c A 7 Present By visual comparison Spectrophotometrically 2.5 0 to 5 2.7 4.3 0 to 5 4.5 5-0 5+ 4.7 5.3 5+ 5.0 5-0 5 4.7 4.3 5 5.0 10.0 10 9.8 10.0 10 + 10.3 10.7 10 - 10.0 10-7 10 + 10.7 16-0 10 to 20 15-3 19-7 20 20.0 20.0 20 20.0 21.0 20 + 19.9 29.8 30 30.0 29.7 30 30.0 40.0 40 + 39.3 40.3 40 41.3 Plus or minus symbols indicate values slightly greater than, or slightly less than, that of the nearest colour standard.FIELD METHOD FOR THE DETERMINATION OF CARBON DISULPHIDE VAPOUR APPARATUS- Glass bubbler-As shown in Fig. 2. -25 i.d. Fig. 2. Diagram of bubbler. (All measurements are in millimetres) IN AIR590 HUNT, MCNALLY AND SMITH: A MODIFIED FIELD TEST FOR [Analyst, Vol. 98 Aspivator-A rubber bulb or other device capable of drawing air through the apparatus at the rate of 125 ml min-l. Colour comparison tubes-Flat-bottomed glass tubes of 10 mm i.d., calibrated with a mark at a height of 50mm. Filter-paper holdev. REAGENTS- Reagents should be of analytical-reagent grade when possible. Absorption solution-Dissolve 0.01 g of copper(I1) acetate monohydrate in a small amount of cold ethanol and transfer the solution into a 500-ml calibrated flask with about 100 ml of ethanol (industrial methylated spirit can be used instead of ethanol if desired).Add, from a measuring cylinder, 5 ml of triethanolamine and wash any residue in the cylinder into the flask with ethanol. Swirl the contents of the flask until the triethanolamine has completely dissolved. Add 2 ml of diethylamine and dilute to 500 ml with ethanol. This solution can be used for 1 month if stored at temperatures below 30 "C in tightly stoppered bottles. Lead acetate solzttiofz-Dissolve 10 g of lead acetate trihydrate in 90 ml of water. Add 5 ml of glacial acetic acid and 10 ml of glycerol to the solution and mix well. Lead acetate $apers--Immerse strips of Whatman 3MM chromatographic paper, 20 mm wide and 100 mm long, vertically for 1 minute in the lead acetate solution contained in a 100-ml measuring cylinder.Remove the papers and allow the excess of liquid to drain. Suspend the papers vertically and allow them to dry at room temperature in an atmosphere free from hydrogen sulphide. When dry, remove and discard portions 25 rnrn in length from the top and bottom of each strip. Store the prepared papers in stoppered, wide-necked, dark-glass bottles to protect them from light and air. Use within 14 days of preparatjon. PREPARATION OF COLOUR STANDARDS- Caybon disulphide solutiow-Dilute 1 ml of carbon disulphide to 100 ml with ethanol (or industrial methylated spirit). Dilute 1 ml of this solution to 250 ml with ethanol. This solution can be used for 1 week if stored below 30 "C in a well stoppered bottle.Prepare four standards representing 0, 10, 20 and 40 p.p.m. V/V of carbon disulphide in air (0, 30, 60 and 120 mg 111-3) by adding 0, 0.30, 0-60 and 1-20 ml of the carbon disulphide solution to 10ml of the absorbing solution. Allow the colours to develop for 10 minutes before carrying out colour comparisons. Some approximations are involved in these standards for the visual determination, but allowance is made for volume changes and for the 94 per cent. efficiency of absorption in the bubblers. An additional standard representing 5 p.p.m. V/V (0.15 ml) can be included if required. The colour standards are stable for about 4 hours if kept in well stoppered tubes. As an alternative, a series of permanent glass standards in a comparator disc, obtainable from Tintometer Ltd., Salisbury (Catalogue No.6/56), can be used in conjunction with a Lovibond 1000 Comparator with a vertical viewing attachment. PROCEDURE- In a carbon disulphide free atmosphere, transfer by means of a pipette 10 ml of the absorption solution into the glass bubbler. Insert the inlet tube and attach the aspirator. Transfer the apparatus to the sampling site and collect 500 ml of the atmosphere. Remove the apparatus to an uncontaminated area and, 10 minutes after collection, transfer the solution into a colour comparison tube, filling the tube up to the calibration mark, and compare its colour with those of the colour standards by viewing downwards through the 50-mm depth of the liquids against a white background in daylight.If the presence of hydrogen sulphide is suspected, connect a suitable paper holder containing a lead acetate paper to the inlet tube of the bubbler. Re-adjust the aspirator, if necessary, so as to sample 125 ml in 1 minute; the resistance of the paper may reduce the volume of air sampled if a rubber-bulb aspirator is used. DISCUSSION AND RESULTS Both the turbidity and change of hue found occasionally with the current Factory Inspec- torate method,l and the change in colour with time observed with the first modificationsAugust, 19731 59 1 to this method, appear to have resulted from the formation of hydrolysed copper species in the solution. The addition of triethanolamine, which formed a stable ammine-type complex, overcame this effect and imparted a much greater tolerance to the presence of water in the absorption solution.An added advantage for field use was that this solution could be prepared and transported as a single reagent instead of the two reagent solutions and additional ethanol that were previously required. The method is capable of giving more accurate results if a spectrophotometer is used and optical densities are measured at 430nm in 10-mm cells. A calibration graph can be prepared for use with atmospheres containing 0 to 40 p.p.m. V/V by adding volumes of 0 to 1-20ml of the carbon disulphide solution to 10-ml calibrated flasks, diluting to the mark with absorption solution and measuring the optical densities after 10 minutes. Two bubblers in series should be used to ensure complete collection of carbon disulphide.In calculating the results, an atmospheric concentration of 20 p.p.m. V/V of carbon disulphide can be taken as equivalent to 62.2 pg 1-1 at 25 “C. One sample of each pair was absorbed in ethanol and returned to the laboratory for spectrophoto- metric determination of carbon disulphide by the xanthate method. The results are shown in Table V. Some of the samples were taken in positions that were not typical of a working environment in order to cover a wide range of atmospheric concentrations and provide a valid test for the method. None of the areas tested that might be occupied by workers had atmos- pheric concentrations of carbon disulphide greater than the threshold limit value3 of 20 p.p.m. V/V.Two samples (not shown in Table V) were taken in the factory in areas where hydrogen sulphide was known to be present. The field testg for hydrogen sulphide showed the presence of 5 and 8 p.p.m. V/V of this gas, which had little effect upon the colours produced by the carbon disulphide test. The use of lead acetate paper reduced the intensity of colour produced in each instance but the evaluation of the atmosphere remained the same, both with and with- out the treated paper, in relation to the nearest visual colour standard because of the low level of interference (approximately one tenth of the threshold limit value) at these concentrations. It was noted that, although the factory was pervaded by a characteristic “sulphur” odour, neither carbon disulphide nor hydrogen sulphide was detected by its odour, despite the fact that the field tests showed one or both to be present.THE DETERMINATION OF CARBON DISULPHIDE VAPOWR IN AIR Tests were conducted a t a viscose factory where duplicate samples were taken. TABLE V TEST OF THE FIELD METHOD AT A FACTORY Hydrogen sulphide, I A > Carbon disulphide concentration, p.p.m. V / V , by Sample p.p.m. V/V xanthate method field method 1 0 2 0 3 0 4 < 2 5 <2 6 0 2.6 4.6 6.4 22.4 25 160 o+ 20 to 40 (25) 0 to 5 (5-) 5 20 to 40 (20f) > 80 Figures in parentheses indicate visually estimated values. Plus or minus symbols indicate values slightly greater than, or slightly less For sample 6, 250 rnl of atmosphere were taken. than, that of the nearest colour standard. The sensitivity of the method was improved by reducing the concentration of copper(I1) acetate in the absorption solution, which gave a colourless reagent blank, and by increasing the path length of light used in viewing the colours.The stability of the absorption solution was improved by the addition of triethanolamine. The results shown in Tables IV and V show that the modified method is an effective replacement for the earlier method. This work was undertaken on behalf of the Department of Employment Committee on Tests for Toxic Substances in Air. The authors are indebted to the Government Chemist for permission to publish this paper, and to H.M. Factory Inspectorate for arranging the field tests.592 HUNT, McNALLY AND SMITH REFERENCES Department of Employment, “Methods for the Detection of Toxic Substances in Air, Booklet No. Tischler, N., Ind. Engng Chem. Analyt. Edn, 1932, 4, 146. Department of Employment, “Threshold Limit Values for 1971,” Technical Data Note 2/71, Sonnenschein, W., and Schafer, K., 2. analyt. Chem., 1953, 140, 15. Brown, E. G., Analyst, 1952, 77, 211. Viles, F. J., J . I n d . Hyg. Toxicol., 1940, 22, 188. Vasak, V., in Brieger, H., and Teisinger, J., Editors, “Toxicology of Carbon Disulphide (Inter- Jacobs, M. B., “The Analytical Chemistry of Industrial Poisons, Hazards and Solvents,” Second Department of Employment, “Methods for the Detection of Toxic Substances in Air, Booklet Received January 18th, 1973 Accepted March 2nd, 1973 6, Carbon Disulphide Vapour,” H.M. Stationery Office, London, 1968. H.M. Factory Inspectorate, London, 197 1. national Symposium),” Excerpta Medica Foundation, London, 1967, p. 15. Edition, Interscience Publishers Ltd., London, 1949, p. 334. No. 1, Hydrogen Sulphide,” H.M. Stationery Office, London, 1969. 1. 2. 3. 4. 5. 6. 7. 8. 9.

ISSN:0003-2654    

DOI:10.1039/AN9739800585

出版商:RSC    

年代:1973

数据来源: RSC

摘要:

Analyst, August, 1973, Vol. 98, @. 593-595 593 Determination of the Antioxidant 1,3,5 -Trimethyl-2,4,6-tri(3’,5’-di- t-butyl-4’- hydroxy- benzy1)benzene in Feeds BY G. F. BORIES (Station Centrale de Nutrition, Centre National de Recherches Zootechniques, 78- Jouy-en- Josas, France) 1,3,5-Trimethy1-2,4,6-tri( 3’, 5’-di-t-butyl-4’-hydroxybenzyl) benzene (Ion- ox 330) is an antioxidant that is used for the preservation of plastic food wrappings. Its use as a fat stabiliser could be extended to animal feeds. By using the method described, 200 p.p.m. of Ionox 330 in a feed can be determined with good reproducibility. The method consists in extracting the Ionox 330 with chloroform, purifying it by using thin-layer chromato- graphy, and measuring at 522 nm the colour developed in the presence of iron (111) chloride and 2,2’-bipyridyl.Possible interference from 2,6-di-t- butyl-p-cresol (butylated hydroxytoluene; BHT) can be avoided by using this chromatographic technique. THE use of 1,3,5-trimetliyl-2,4,6-tri(3’,5’-di-t-butyl-4’-hydroxyb~nzyl)benzene (Ionox 330”) is authorised in France and several other countries as a stabiliser for the plastic wrappings that are used for packing foods. This substance, which has a high relative molecular mass, is not absorbed in the intestine, thus making it attractive as an antioxidant for stabilisation of fat in animal feeds. A method for determining it in lard, which involves the use of gas- liquid chromatography, has been proposed.1 The high boiling-point of this substance (above 300 “C) presents severe drawbacks, especially the need to heat the chromatographic column to 330 “C. Consequently, the column gives uncertain reproducibility and has a limited life because of fats carried over during the extraction.In order to avoid these difficulties, we tried a colorimetric method of determination that involved the use of Emmerie and Engel’s non-specific reaction2 after purification by thin-layer chromatography. EXPERIMENTAL EXTRACTION OF IONOX 330 FROM THE FEED- Several solvents were tried, but complete extraction could be achieved only by boiling the feed with chloroform under reflux. An aliquot of the extract was concentrated to one tenth of its volume by evaporation in a vacuum before being purified by thin-layer chromato- graphy on silica gel. In order to prevent oxidation of Ionox 330 during the extraction and concentration steps, hydroquinone was added to the feed just before extraction.If this precaution is not taken, as much as 20 per cent. of it may be destroyed in several hours. PURIFICATION OF THE EXTRACT- The concentrated extract is purified by thin-layer chromatography under the following conditions: chromatography on silica gel activated for 30 minutes at 105 “C; elution with cyclohexane - toluene mixture (3 + 2); and simultaneous chromatography of some of the same extract so as to produce indicator spots, which are rendered visible by spraying them with potassium permanganate or antimony(V) chloride solution. The corresponding area of unsprayed plate is scraped off and extracted with chloroform. OUTLINE OF METHOD OF DETERMINATION- We attempted to find a very sensitive colorimetric method for the determination of Ionox 330 that had been purified by thin-layer chromatography. We adapted Emmerie and Engel’s method, which has been used with success for determining several antioxidants3 and is based on the reduction of iron(II1) chloride to iron(I1) chloride and reaction of the latter with 2,2’-bipyridyl to give a red-coloured complex that absorbs at 522 nm.The reaction is * Shell Chemical Company registered trade name. @ SAC and the author.594 BORES : DETERMINATION OF 1 ,Q,G-TRIMETHYL- [Analyst, Vol. 98 carried out in methanolic solution, the colour development taking 30 minutes, after which time the colour is measured. In order to use the chloroform extract of the silica gel adsorbent directly, we carried out the colour development in chloroform - methanol (1 + 1) for 30 minutes.A graph was drawn representing the colour intensity obtained after 30 minutes for a given Ionox 330 concentration in terms of the value of the methanol to chloroform ratio (Fig. 1). It shows that a small variation in the 1 : 1 value for this ratio results in large variations in the reading, and the ratio must therefore be rigidly controlled in order to obtain consistent results. The 2,2'-bipyridyl - iron(I1) complex has a high specific absorption at 522 nm ( E = 3 x lo4). By plotting optical density against concentration of Ionox 330 at concen- trations from 5 to 15 pg ml-l a calibration graph was prepared, which passed through the origin.By using this graph, 5 pgml-1 of Ionox 330 can be accurately determined and 0.5 pg ml-l detected. 0 1 2 3 4 Methanol to chloroform ratio Fig. 1. Effect of the methanol to chloroform ratio on the intensity of the colour developed after 30 minutes with 10 pg ml-l of Ionox 330 METHOD APPARATUS- were used. REAGENTS- Desaga thin-layer chromatographic apparatus and a Jobin and Yvon spectrophotometer Silica gel-Kieselgel G, Merck. Antimony( V ) chloride solution, 25 per cent. VlV in cyclohexane. Potassium permangunate solution, 1 per cent. MethanoZ-Analytical-reagent grade. 2,2'-Bipyridyl (Eastman Kodak) solution, 2 per cent. m/V in metlzafiol-Keep the solution in a cold and dark place. Iron(III) chloride solution, 2 per cent. m/V in methanol. PROCEDURE- Weigh 10 g of a feed containing 200 p.p.m.of Ionox 330 into a 250-ml round-bottomed flask, add 0-5 g of hydroquinone, then extract the mixture by boiling with 100 ml of chloro- form under reflux for 10 minutes. Cool, then filter the mixture on a filter-paper and evaporate 50 ml of the extract contained in a flask down to about 1 ml in a vacuum. Transfer the concentrate with a pipette into a 5-ml vial, rinsing the flask several times with small portions of chloroform, and make the volume up to 5 ml. With a microsyringe, place 0.5 ml on a plate (20 x 20 cm) coated with a 0-5 to 0-75-mm thick layer of silica gel and activated at 105 "C for 30 minutes. Arrange margins on each side of the plate that are wide enough to enable a spot of the same extract to be placed on each margin so as to assist in localisingAugust, 19731 2,4,6-TRI(3’,5’-DI-t-BUTYL-4’-HYDROXYBENZYL)BENZENB IN FEEDS 595 the site of the Ionox 330 after chromatography.Place the plate in a tank containing the cyclohexane - toluene mixture (3 + 2). After migration of the spots is complete (30 to 80 minutes), dry and render the two spots in the margins visible by spraying with either the antimony(V) chloride or permanganate solution, protecting the centre of the plate by covering it with a glass plate. Outline and scrape the area of the silica gel strip containing the anti- oxidant, and transfer the powder into a small chromatographic column. Elute the column with chloroform and collect the eluate in a 10-ml graduated tube (or vial), ensuring that the volume of eluate does not exceed 5 ml (which is sufficient for complete elution of the Ionox 330), then adjust the volume to exactly 5 ml with chloroform (this constitutes tube X); 5 ml of chloroform in a 10-ml tube (or vial) constitutes the blank solution (tube T).A 5-ml volume of chloroform containing 50 pg of Ionox 330 and another 5-rnl volume containing 100 pg of Ionox 330, placed in two 10-ml tubes, constitute the standard range (tubes A and B, respectively). In subdued light, add to each of the four tubes A, B, X and T, 1 ml of iron(II1) chloride solution and 1 ml of 2,2’-bipyridyl solution; shake the tubes, then complete the volumes to LO ml with methanol. Allow the colour to develop for 30 minutes, then read the absorption at 522 nm as rapidly as possible, with tube T as a reference.RESULTS AND DISCUSSION The method described has been used for the determination of Ionox 330 contained in several types of feeds. The product was incorporated in the proportion of 200 p.p.m. To test the reproducibility of the method, a series of determinations was performed on a control feed to which were added, just before extraction, 200 p.p.m. of Ionox 330 dissolved in a small volume of chloroform. The results shown in Table I were obtained from ten successive determinations on each of the feeds. TABLE I RESULTS FOR RECOVERY OF IONOX 330 FROM FEEDS Ionox found in control rat feed with 200 p.p.m. added* 192 184 178 184 182 186 194 184 182 182 Average . . .. . . 184.8 Average recovery, per cent. 92.5 Coefficient of variation . . 2.6 Ionox 330 found in feeds stated to contain 200 p.p.m.of this additive, p.p.m. Rat feed Highly pigmented - for laying hens Batch I Batch I1 (lucerne meal) 200 200 192 216 191 216 203 189 184 200 176 182 210 190 186 200 186 176 196 178 194 190 178 204 186 178 184 196 194 180 199.7 186 189-8 100 93 95 4.4 4.4 6.4 I Pig feed 196 192 212 178 184 180 206 200 188 182 192 96 5.6 * Added in solution. The recovery rate of the additive is satisfactory. It should be noted that the presence of pigments (from lucerne meal) does not affect the percentage recovery. A good separation of Ionox 330 from interfering substances, as well as from 2,6-di-t-butyl-~-cresol (butylated hydroxytoluene; BHT), can be obtained by using the thin-layer chromatographic method described. Finally, by making use of thicker silica gel layers with possibly several elutions with the same solvent system, it should be possible to chromatograph larger amounts of the extract and thus determine 50 p.p.m. of Ionox 330 in a feed with equal precision. We are grateful to the Compagnie de Produits Chimiques Shell (Paris) for permission to publish this paper, and we thank Mrs. E. Biette for technical assistance. REFERENCES 1. 2. 3. “Determination of Ionox 330 in Lard,” Shell Chemical Company, unpublished report. Emmerie, A., and Engel, C., Red Trav. Chim. Pays-Bas Belg., 1938, 57, 1351. Mahon, J. H., and Chapman, R. A., Analyt. Chew., 1951, 23, 1116.

ISSN:0003-2654    

DOI:10.1039/AN9739800593

出版商:RSC    

年代:1973

数据来源: RSC